If my ex and I were legally separated, and bills came in for our sons hospital visits that I never received nor knew about, am I legally responsible for them. They were never paid, went to collection and I now have a judgement against me in small claims court.

Why would you think they weren't? YOU were responsible for giving them updated mailing addresses... that falls on you, not them. And it doesn't give you an out for not paying the bills you knew you were incurring.

The thought you never got the bills after the service never crossed your mind? You never thought to ask them?

Sorry... these were your bills... and your responsibility.