Unable to switch child Apple ID on iPhone and iPad after creating a new account

We're trying to add the new child Apple ID to an iPhone and and iPad and it won't accept the new email associated with the child account, possibly because the email on the devices already is the previous Apple ID associated as an adult.

I tried to add the child Apple ID as a secondary so I could make it the primary and then delete the adult account, but nothing is working. Please help!

[Re-Titled by Moderator]

Original Title: My daughter was listed as adult. I created a new Apple ID, now I can't change her login on her devices