Disable Family Sharing

According to the last paragraph in this link:

Family Sharing and Apple ID for your child - Apple Support

Your child must remain part of your family group until they turn 13.* However, if necessary, you can transfer a child to a different family group. You can also create Apple IDs for children through Apple ID for students.

I don't think you will get much more help in this forum. If you still have questions, I would contact Apple Support:

Contact Apple for support and service - Apple Support