In older games like Deus Ex and Fallout 2 you can literally blow kids to pieces. In the Hong Kong levels in Deus Ex one of the mobs' leaders even comments on a certain kid's death if you kill him.
I'm not trying to sound like a "I hate kids because I am dark and edgy and filled with a hateful soul" kind of guy, but I really don't see why they did away with the option to kill kids in games. It was pretty annoying in Fallout 2 when kids would pickpocket and steal your items or in Deus Ex when they threatened and insulted you, but luckily you were able to do something about it. It's pretty jarring in Skyrim or New Vegas or what have you when some kid's being a knob and there's nothing you can do about it. Like, not even a dialogue option to tell them to be quiet or something.
Like you said there's not really a difference between killing a bunch of pixels that look like a kid and killing a bunch of pixels that look like anything or anyone else, so I don't get why it matters. Games that don't allow you to kill kids (which is most of them) aren't really any worse off for it, though.