JUSTICE TURNER delivered the opinion of the court:

Respondent, Michael R. Case, appeals the September 30,2003, order of the McLean County circuit court, awarding petitioner,Courtney L. Case (now known as Courtney Mammenga), a $35,849 judgmentfor back child support. Specifically, Michael asserts the trialcourt erred in finding Courtney was equitably estopped from seekingback child support only for the period of April 12, 1999, to December19, 2001. We affirm.

The parties were married in June 1990 and have two children together, Jordan (born in February 1990) and Mackenzy (born inJune 1991).

In June 1994, Courtney filed a petition for dissolution ofmarriage. She was represented during the divorce proceedings by E.William Rolley. In August 1994, the trial court awarded Courtneytemporary custody of the minor children and ordered Michael to pay$400 in child support every two weeks. In December 1995, the courtentered a judgment of dissolution on grounds only.

In July 1996 and July 1997, Courtney filed petitions for arule to show cause, contending Michael's child-support payments werein arrears. In November 1997, Michael filed a petition to modifychild support, asserting he was a full-time student and had no job. In September 1998, the trial court entered a supplemental judgmentorder, ordering Michael to pay $200 per month in child supportstarting September 5, 1998, and expressly reserving the matter of thechild-support arrearage.

On April 12, 1999, the parties entered into an agreement(Agreement), under which Michael was to sign a consent to adoptionand waiver of parental rights for each of the minor children and, inconsideration, Courtney would not seek to collect current or backchild support. That same day, Michael signed a consent to adoptionfor both of the children. In June 1999, Courtney married BrianMammenga.

In March 2000, Courtney executed a limited power ofattorney and authorization to release information to CSE ChildSupport Enforcement, Co. In June 2000, that organization requestedcopies of the court orders in this case. In October 2001, theIllinois Department of Public Aid (Department) began filing noticesto withhold income for child support to several of Michael'semployers. In December 2001, a $200 child-support payment waswithheld from Michael's paycheck.

In November 2002, the Department filed a petition foradjudication of indirect civil contempt, arguing Michael owed$32,956.81 in back child support. That same month, Michael filed amotion to establish specific visitation times with the minorchildren. In February 2003, Michael filed a motion to dismiss theDepartment's petition, alleging Courtney could not seek payment ofcurrent and back child support because of the Agreement.

In April 2003, the trial court granted Courtney leave towithdraw the contempt petition and to file an alternative pleading. Courtney then filed a motion for judgment, seeking a judgment of$32,856.81 plus interest for back child support. That same month,Courtney and her husband filed a petition for adoption, No. 3-AD-21(Circuit Court of McLean County). In May 2003, Michael filed amotion to dismiss Courtney's motion for judgment, again asserting shewas estopped from seeking support because of the Agreement.

On May 20, 2003, the trial court held a joint hearing onMichael's motion to dismiss and Courtney's motion for judgment. GuyParr, superintendent of Heyworth Community Unit No. 4, testified theminor children were first enrolled in his district in the fall of1999 with the last name Mammenga. The children were registered underthat name until the fall of 2002 when Michael informed Parr thechildren's legal name was Case.

Lori McCormick, deputy director of juvenile court servicesfor McLean County, testified one of her job responsibilities was towitness consents to adoption. She recognized her signature as awitness on the consents to adoption executed by Michael on April 12,1999. Her records indicated Michael's appointment had been set up byRolley. She did not know what happened to the original copies of theconsents.

Michael testified Courtney approached him about havingBrian adopt the children so they could have his last name. On April12, 1999, Michael, Courtney, and Brian met at Rolley's office, andMichael agreed to sign the consents if he would not be responsiblefor past or current child support. Rolley drafted an agreement,indicating Courtney would not seek to collect any current or backchild support in consideration of Michael signing the consents toadoption. Michael and Courtney both signed the letter despite Rolleynoting his disagreement with the letter. The parties then went tojuvenile court services, and Michael alone met with McCormick. Hethen signed the consents to adoption, and McCormick kept theoriginals.

After signing the consents, Michael did not visit thechildren, did not talk with Courtney, and just assumed Courtney hadfollowed through with the adoption. On December 12, 2001, Michaelhad a $200 child-support payment withheld from his paycheck. InApril 2002, Michael spoke with Courtney and learned the adoption hadnot been completed. Michael then resumed visitation with thechildren. He testified he would have revoked the consents if the lawpermitted him to do so.

Courtney testified Rolley had the agreement alreadyprepared when the parties arrived at his office. After they signedthe Agreement, Courtney left with Brian and did not go with Michaelto the courthouse. Courtney stated she was unaware that Michael hadgone to the courthouse and executed the consents. According toCourtney, Rolley never informed her that Michael had signed theconsents, and she was first aware of the consents when Michael filedhis motion to dismiss. Courtney had not gone through with theadoptions because of the costs of an adoption without a consent.

Brian testified his adoption of the children had beenfinalized the day before the hearing. He also testified Courtneyleft Rolley's office with him.

In September 2003, the trial court entered an order, (1)denying Michael's motion to dismiss; (2) finding Courtney wasestopped from receiving child support and interest accruing fromApril 12, 1999, to December 19, 2001; (3) concluding child supportaccrued through May 20, 2003, on which date child support terminated;and (4) awarding Courtney a $35,840 judgment for back child support. This appeal followed.

On appeal, Courtney, the appellee, has not filed abrief. A reviewing court is not compelled to serve as an advocatefor the appellee and is not required to search the record for thepurpose of sustaining the trial court's judgment. However, if therecord is simple and the claimed errors are such that the court caneasily decide them without the aid of an appellee's brief, the courtshould decide the merits of the appeal. On the other hand, if theappellant's brief demonstrates prima facie reversible error and thecontentions in the brief find support in the record, the trialcourt's judgment may be reversed. First Capitol Mortgage Corp. v.Talandis Construction Corp., 63 Ill. 2d 128, 133, 345 N.E.2d 493, 495(1976).

Michael contends the trial court erred in limiting itsapplication of equitable estoppel to the child support and interestthat accrued between April 12, 1999, and December 19, 2001. We willnot disturb such a finding unless it is against the manifest weightof the evidence. See In re Marriage of Smith, 347 Ill. App. 3d 395,401, 806 N.E.2d 727, 732 (2004).

On April 12, 1999, when Michael signed the consents toadoption and the Agreement, he owed Courtney child support that hadalready accrued. Around December 19, 2001, Michael had $200 withheldfrom his paycheck for child-support payments and made some child-support payments thereafter. Thus, the trial court's order requiredMichael to pay (1) the child support that had accrued before April12, 1999, and (2) any unpaid support that had accrued from December19, 2001, to May 20, 2003, the day after the adoption. Since theabove time periods have different considerations, we will addressthem separately.

As stated, Michael owed an unspecified amount of backchild support when he signed the agreement and the consents on April12, 1999. The Agreement provided that Courtney would not seekpayment of that back child support.

Equitable estoppel applies when a person, by his or herstatements or conduct, induces a second person to rely, to his or herdetriment, on the statements or conduct of the first person. Smith,347 Ill. App. 3d at 399, 806 N.E.2d at 730. The party assertingestoppel must have relied upon the other person's acts orrepresentations and have had no knowledge or convenient means ofknowing the facts, and such reliance must have been reasonable. Smith, 347 Ill. App. 3d at 399, 806 N.E.2d at 730-31. However,extrajudicial agreements modifying support are unenforceable. Smith,347 Ill. App. 3d at 399-400, 806 N.E.2d at 731.

As our supreme court has explained, courts have theexclusive authority to modify child support and are not bound by theparties' agreements as to child support. Blisset v. Blisset, 123Ill. 2d 161, 167, 526 N.E.2d 125, 127-28 (1988). In deciding child-support matters, a court must protect the children's best interests. Blisset, 123 Ill. 2d at 167, 526 N.E.2d at 128. To allow formerspouses to modify a child-support order by creating a new agreementbetween themselves without judicial approval would circumventjudicial protection of the children's interests. Blisset, 123 Ill.2d at 167-68, 526 N.E.2d at 128. Former spouses could agree tomodify child-support obligations to benefit themselves whileadversely affecting their children's best interests. A parent cannotbargain away his or her children's interests. Thus, parents maycreate an enforceable agreement for modification of child supportonly by petitioning the court for support modification and thenestablishing, to the court's satisfaction, the parties' agreement isin accordance with the children's best interests. Blisset, 123 Ill.2d at 168, 526 N.E.2d at 128.

In Smith, 347 Ill. App. 3d at 397, 806 N.E.2d at 729, thecustodial parent sought payment of overdue child-support payments. The noncustodial parent asserted the custodial parent was equitablyestopped from recovering the arrearage because she had agreed towaive the child-support payments as long as he purchased items forthe children. Smith, 347 Ill. App. 3d at 399, 806 N.E.2d at 730. While the Second District affirmed the trial court's finding that nosuch agreement existed, it further noted that, even if the agreementdid exist, it was unenforceable since the parties did not obtainjudicial approval. Smith, 347 Ill. App. 3d at 400-01, 806 N.E.2d at731. Thus, the Second District concluded the trial court's rejectionof the equitable estoppel defense was not against the manifest weightof the evidence. Smith, 347 Ill. App. 3d at 401, 806 N.E.2d at 732.

In this case, the parties' agreement was unenforceablesince they sought to usurp the authority of the court by modifying acourt-ordered support obligation themselves. See Smith, 347 Ill.App. 3d at 401, 806 N.E.2d at 732. Additionally, the child supportthat had already accrued at the time the parties signed the agreementcould not have been modified even by a court. See In re Marriage ofCorkey, 269 Ill. App. 3d 392, 397, 645 N.E.2d 1384, 1388 (1995)(noting past-due child-support installments are vested rights that acourt cannot modify retroactively). Accordingly, the trial court'saward of back child support that had accrued prior to April 12, 1999,was not against the manifest weight of the evidence.

While the agreement itself is unenforceable, Michael didexecute consents to the adoption of the children, and the trial courtconcluded he had proved equitable estoppel until December 19, 2001. On that date, the withholding of child support put Michael on noticethat child support was being collected, ending his reasonablereliance. The court then terminated the child-support payments whenthe children were adopted.

That finding is consistent with Bodine v. Bodine, 141 Ill.App. 3d 21, 489 N.E.2d 911 (1986), cited by Michael. There, thenoncustodial parent executed a consent to the adoption of the minorchildren, but the custodial parent decided not to go through with theadoption. Bodine, 141 Ill. App. 3d at 22, 489 N.E.2d at 912. Thetrial court applied the doctrine of equitable estoppel and found thecustodial parent was not entitled to child support for the periodafter the consent was signed to when she filed her criminalnonsupport action. Bodine, 141 Ill. App. 3d at 22-23, 489 N.E.2d at912. On appeal, the Third District affirmed the court's ruling,finding the nonsupport action should have adequately informed thenoncustodial parent or at least triggered further investigation intowhether his parental rights would be terminated. Bodine, 141 Ill.App. 3d at 23, 489 N.E.2d at 913.

In this case, Michael began investigating whether thechildren had been adopted after the child-support payment waswithheld from his income around December 19, 2001. Upon learning theadoption had not taken place, Michael resumed visitation with thechildren and voluntarily made some child-support payments. Thus, wefind the trial court's conclusion the period of estoppel ended onDecember 19, 2001, was not against the manifest weight of theevidence.

For the reasons stated, we affirm the trial court'sjudgment.

Affirmed.

STEIGMANN and APPLETON, JJ., concur.