Managing personal Outlook.com account settings, security, and privacy
Thank you for the information and I understand.
If the account has not been signed in for more than 2 years, I'm afraid your child's account may have already been deleted/closed due to inactivity.
As stated by Microsoft, accounts that have not been signed in or have been locked for more than 2 years will be deleted due to inactivity, and they can no longer be recovered: https://support.microsoft.com/topic/microsoft-account-activity-policy-7c0a9fa7-0982-b7c6-fd72-df852b63699f
Please let me know if you need anything else.
Best regards,
Darhyl