Remove child account from family share

If they are under 13 years old, you need to transfer them to another family.

Family Sharing -  Transfer a child to a different family group - Apple Support 

If 13 years old or older, see the article blow.

Family Sharing - Leave.

If necessary, contact Apple ID Support for assistance.  See the Apple Support articles below for contact information.  

Apple ID - Official Apple Support.    

Contact Apple for help with Apple ID account security - Apple Support.   (Link doesn’t always load)  

Get Support          Work through the screens to get a chat option. You may have to pick a slightly different problem.

Apple Support schedule phone call

If necessary, try Twitter

https://twitter.com/AppleSupport.          

From the Apple Support article below. 

Without proof of your identity via security questions and other carefully selected criteria, Apple Support can't help you reset a password or perform any other actions on your account. These policies are audited and reviewed on a regular basis.

Security and your Apple ID - Apple Support.