change a child's apple id password

See the Apple Support article(s) below.

 Apple ID Password Change.      

 What to do after you change your Apple ID or password - Apple Support     I suggest doing this before you change your Apple ID or password.

See Family Privacy Disclosure for Children https://www.apple.com/legal/privacy/en-ww/parent-disclosure/. Toward the end of this document you see: this heading Access, Correction and Deletion. Here is a quote from it.

"If at any time you want to access, correct or delete data associated with Family Sharing or your child's Apple ID, please contact us here: www.apple.com/privacy/contact/."