Hello Brian,
The good news is that since the account is part of your Microsoft Family group, you should be able to reset the password directly from your parent (organizer) account. Just go to account.microsoft.com/family, sign in with your main Microsoft account, and look for the child’s profile. From there, you should see an option to manage their account or reset their password.
If that doesn’t work, you can also try the standard password reset process at account.live.com/password/reset using the child’s email address, Microsoft may ask you to verify your identity as the family organizer.
This might help
https://support.microsoft.com/en-us/account-billing/reset-a-forgotten-microsoft-account-password-eff4f067-5042-c1a3-fe72-b04d60556c37