Even though the kids have been zombie-kids, that head-shot on summer in the pilot was pretty brutal. Same with penny getting owned by Michonne. They haven't straight up killed a living kid, but Carl was shot and that wasn't exactly 'clean' - it was pretty Gorey. They showed the wound and taking the bullet fragments out and all that. I think if they wanted they could kill a living kid on screen, but that wouldn't really be making history. I don't watch a whole lot of TV, but has a baby been killed ON-SCREEN, without clever little tricks to dampen the effect? Like a straight up kill. That could be it. To be honest it's probably not even a death. I think it will be something completely different and unexpected.