How to activate a locked Apple ID account without access to the primary email address.

My 9-year-old daughter's Apple ID account got locked after I tried to change the primary email address from @gmail.com to @outlook.com, because she no longer has access to the Gmail account. I went through the account recovery process on the Apple ID website. Apple responded that I would be notified at the @gmail.com address, but I can’t access that email. What should I do?