No, actually, if I recall correctly, you can request a password change without using your email on Netflix. You can enter a number to receive a text. Getting that notification is just the after the fact warning. So the teen reset the password, and the OP was informed of that by automated email.
But, of course, the change isn't permanent. I'm actually not sure how you would get locked out of your own account- you can just change the password again.
Maybe you could change security questions or something, but the easiest way to stop the hacking is to get rid of that account and use a whole different username. Or OP could erase the payment information, abandon the account, and it would be useless. (I "hack" my own accounts for stupid reasons like expired debit card. The account does not work without a valid form of payment on file).
There's no malicious reason I can think of to hack someone's Netflix account. I mean, yeah, to watch it. But other than that, no. Which is probably why it doesn't have better security.