Unable to transfer child to another family share

I’m trying to transfers child account under 13 yrs to my family share. When i send the invite it pops up on the other device that he is on and part of the family share acct. It starts to work but then comes back saying he is already in a family share.

The family organizer that he is on does not give an option to transfer the child. He does not have an Apple Cash acct or sharing any apps. I don’t want to delete his acct but don’t know what else to do.