Move a child to another group using Family Sharing - Move a child to another group using Family Sharing - Apple Support - "While you can't remove a child under 13 years old from your family group, you can move them to another group."
However, it sounds like you aren't even trying to do that but simply have her sign out of her iPad. As was noted before, the hardware doesn't actually matter, it's her account that always has to be part of Family Sharing For that matter she could even not be signed in on any hardware at all but she must be part of a Family Sharing group, uninterrupted. Make sure she is simply signing out of her Apple Account/ID, not Family Sharing itself.
Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support
If you try to create a new FS account it will make things a lot more complicated and may not even have the desired result. If the above tips and documents do not help then contact Apple about this.
Contact Apple. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world."
Click here --> Contact Apple for support and service - Apple Support