How to recover child account after hacker changed the email?

I wish I had better news for you, this forum is just a user to user forum, no-one here has any account access or the ability to recover an account, when a Microsoft account is hacked, if the hackers change the associated data like the Email or if 2FA is enabled on the account, then that account is usually no longer recoverable, try using the process they provide on this link to recover a hacked account, if this process is not successful, sadly, that account cannot be recovered.

https://support.microsoft.com/en-us/account-billing/how-to-recover-a-hacked-or-compromised-microsoft-account-24ca907d-bcdf-a44b-4656-47f0cd89c245