You miss out on a quite nice ring, (+ a random encounter with a cannibal, and some comments from the guards about your smell), but it doesn't affect the main quest. If you're the cheating type you can use the console to get the ring.
I think all characters important to the main quest (and children) are marked as essential, which means they can't be killed. Not sure if they can die after they've played their part, though. They probably can. If you want to harm/pickpocket/drink blood etc from children, you have to use a mod, that allows you to do that.