Can't reset password

After creating the new accounts, you can add your phone number and email address to the accounts at:
https://account.microsoft.com/profile

Because Microsoft does not allow this. Parents cannot change children's passwords.

Why can't I reset this from the parent account?

The best thing I would suggest you to do is to add a phone number or an alternative email address on the accounts. It can be your phone number and email. This way if you forget the kid account email address or password, you can use your phone or email to receive a reset link and will not even need the recovery form.

I'm sorry, on my first reply I sent you the link to recover the account email, not password. This is the correct one:

https://support.microsoft.com/en-us/account-bil...

But if nothing there works, I'm sorry but you won't be able to recover it.