You shouldn’t be able to kill the kids. First of all killing children is wrong but the reasons people are giving don’t cut it.
1. Because they’re annoying brats. C’mon, be honest. That’s not a good enough reason.
2. It makes the game more realistic. “I see murder as murder in technical terms, and if Skyrim were aiming to be more realistic, then kids should be like real life kids and able to die.” If that’s the case then if you murder anyone and get caught you should go to jail or be executed. And not a jail that you can pick your way out of. And if you want it really realistic if you’re executed the game would delete your save file and any back-ups you’ve made. There are a lot of ways the game could be made more realistic without adding killing the kids. Should they also add raping the women? (Not suggesting it. Just making a point.) That being said, I think I’d be okay with Bethesda killing the kids if it were integral to the plot. (You have to avenge their death. They were killed along with the rest of the town. That kind of thing.)
3. “Plus, they may be kids now, but sooner or later they might become mercenaries or soldiers. Killers. Won't think they're so cute then.” The key word here is “might”. You can’t go around killing the kids on the chance that they will turn bad.
On a side note, I also think you should be able to kill quest givers. You mess up and kill someone you shouldn’t? Too bad for you. I also think they should throw in some innocents so that you can’t just walk into a dungeon and start killing everyone you see. “Those weren’t bandits. They were just some traders getting in out of the rain.”