You have nothing to lose by reporting it to Sony by phone, but given that the sign-in information was disclosed to someone else, Sony has no responsibility to do anything. To make matters worse, if you don't know the security information for the account (date-of-birth and security question answer), and there was never a valid e-mail address, then there is really no way for you to prove that the account belongs to you. It could have been created by anyone. Sony is accustomed to kids lying about their age to get access to age-restricted games and content, and it has been fairly tolerant of that. If you can remember the answer to the security question that was selected for the account, then you might have a chance. Otherwise, be prepared to start over with a new account. One of the advantages of using a sub-account, which kids are required to do anyway, is that the parent can always recover the account if it is stolen.
This, it would sound like YOU are the one trying to steal someone's account instead of getting it back.