Difficulty Transferring Child Under 13 in Family Sharing

I'm trying to get a Child under 13 Apple Id into another Family Share. Every-time they try and add the child's apple id they get the error " Already a member of another family."

So, can someone give me step by step instructions for the other Family Share member to send the proper invite to be able for me to transfer the Child under 13 apple id? Does the invite come to my apple id or the child's apple id???

Any help is appreciated !