Removing child under 13

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

Apple ID Support

Apple ID Support (2) 

Apple Support by e-mail or chat    Work through the screens to get a chat option.  You may need to pick a problem that is slightly different than yours to get a chat option.  

Apple Support using Twitter

Or fill out and submit this form.          The form is for iTunes, but it will get passed to the ID Support section.  

You can only move them to another family account. If that's not an option, you'll need to contact Customer Support. Use the link at the top right of every page of these forums.