Email used by Another Apple ID Error

Through Family I added my child to get her an Apple ID (using a newly created email address). The set up went great and now they are part of my “family”. However, when I went to change the Apple ID on an old iPad to give them I received an error message that states “email used by another Apple ID”. This is not possible because it is a new email and Apple ID that has never been used. I considered deleting her from the family and starting over but because she is a minor I can not delete it. Any suggestions on how I can change the Apple ID on the iPad?