Hello Mombyone,
Thank you for posting in the Apple Support Communities, and for reviewing the related article Move a child to another group using Family Sharing. We'll get you the best options.
For this issue, we think the info in the section "If you can't accept the invitation" in the support article below might provide additional insight:
Set up Family Sharing
If the child hasn't moved to a different family group yet in the last year, try restarting the devices and ensuring they're fully updated to the latest software version. If an update is available, be sure they're backed up first.
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
Back up your Mac
Update macOS on Mac
If this concern continues after taking the steps above, our resolution is to contact Apple Support directly. Depending on your region, you may be able to reach them here:
Get Support
We hope this information is useful. Kind regards.