Family Sharing

You can try the below. If that doesn't work, contact Apple ID Support.

Make sure you sign out from the places in the 4th link before deleting the ID.

How to delete your Apple ID account

Apple ID – How to Delete

Delete Apple ID (new)

What to do after you change your Apple ID or password - 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.  

Hey,

You can't remove a user that is under 13 years old - you can tranfer the user to another family sharing group.

This link might be helpful : https://support.apple.com/en-il/HT201080.

best regards