Thre are a lot of pieces needed to make this work properly, so PLEASE bear with these Y/N questions:
Does your child have their own AppleID as part of a larger Family Sharing group, w/ an adult having a different AppleID as the "Organizer"
Set up Family Sharing - Apple Support
Was the child's Apple ID created by the organizer by THIS process ? (Note that the most critical steps are performed from the organizer's phone, not the child's.)
Create an Apple ID for your child - Apple Support
Does your child's AppleID accurately reflect their birthdate?
Did the organizer personally setup the screentime on the child's phone and correctly select "This is a Child's Phone" (vice "This is My Phone")
Was a unique Screentime Passcode set -- different from the child's device passcode and known only to the adult?
Was the Screentime Passcode linked to the adult's AppleID to allow recovery?
If it was linked, might the child know the Adult's AppleID and Password?