Hello! Do you still have access to the recovery email or phone number added to your child’s Microsoft account?
Try to reset the password using Microsoft’s official recovery page at https://account.live.com/password/Resetchoose Forgot password, enter your child’s email, then complete the security check.
If recovery does not work because there is no recovery info or history, contact Microsoft Support so they can verify ownership and review the account through https://support.microsoft.com/account-billing