My son's account has the email and password changed, tried online recover but no luck for now

I have two child's account under my main account. For both I purchased a license for Minecraft. Now my son, yesterday received an information that his email linked to the microsoft has changed from ***@gmail.com to "dnei2031@gmail.com".

When he tries to log in with his original gmail email, the form says "the account doesn't exist". When I try to restore the account with his email, same thing. So I had to put the new mail in the recovery form.

Now, checking his email history I found that the access was granted with a "single use code" with several attempts, one of which must have worked.

Honestly, I'm not sure wether he's been tricked into giving the access code, but now I have this situation:

1. my account is the parent account, and in the family management I have still my son but with the new, fraudolent email.

2. I tried a recovery indicating my account as alternate email, and put in the four letters of the card I used to buy the license.

3. the first attempt at recovery failed saying "there is no enough information to recover this account

4. I have proof of every change has been made in this account and posted another request with my child's account as recovery

Now, I'd like to understand why it is so difficult to restore an account, changed yesterday, that belongs to a child that is in my family.

Also, I don't understand if this way i lost my minecraft license btw. Not really important, but I don't understand exactly why they stole this account given that I can create a new one with the same email.

Isn't there a support email I could write to with some humans evaluating the case? I'm very disappointed on how this is managed by Microsoft, so far.

Please help!

[edit: i forgot to mention that the password is very secure and managed by a password manager]

:)