WHY CAN'T WE REMOVE CHILDREN FROM FAMILY?

Children under 13 can't be removed from a family group. Age varies based on your country or region - according to apple in this link - https://support.apple.com/en-us/HT201080.

You might find contacting apple support as helpful.

If you can't transfer them to another family, you can try deleting one ID.

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     

If necessary, contact Apple ID 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.