I too am not intimately versed in the nuances of Parental Controls …
… but I DO know that a child can ONLY be a member of one family (YOURS).
And even if the “ex” family asks her to enter her credentials as is required to be “invited” into a new family … she still will NOT move w/o YOUR explicit approval of the invitation.
Suggest a thorough review of the following, and “locking down” the phone as far as you think is necessary. Certainly restricting the ability of “the child” to change her AppleID and Password should be a “minimum” starting point.
A strong case could be made that an 8-yo shouldn’t even know her AppleID password at all.
Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support
Move a child to another group using Family Sharing – Apple Support (AU)