Change children's Apple ID

I accidentally created an @icloud.com address for my kid's (under 13) Apple ID, and now I want to change the primary email to his correct address. However, there is no option for setting a new primary email when I sign into his account at appleid.apple.com. The correct address is listed under Alternative Email Addresses I thought maybe I'd have to change it through my account, but I couldn't find any help there either. How can I change his primary email address?

EDIT: The only buttons next to the alternative email address are edit and delete. There are no buttons next to the primary address. Both addresses are verified