change my child's family account password

Why doesn't it work? Error message(s)? More information would help.

If you know the password.

 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.

If you forgot the password, enter the Apple (iCloud) ID and click Forgot Password.

Apple ID - Manage            

Apple ID – Forgot Password      

Here is how to reset a forgotten Apple ID password: If you forgot your Apple ID password - Apple Support

(Family Sharing isn’t involved here.)