The concept of the killer kid is immensely dangerous if mishandled. You can try to be dead serious and have something to say or you can be humorous and break taboos but you can never take it lightly. Of course, Kenny from South Park has died hundreds of times but it's a cartoon so it's easy to shrug away. Set the killings in a realistic setting and give the kid a face, and it's immediately much more difficult to film its death, especially if the kid is the villain of the piece and the hero can only survive by becoming its designated killer. Still, a bunch of films have spun entertaining yarns about children that do nasty things, and here are posters for some of them. If you decide to watch any of these, be careful not to lose sight of the fact that it's all just fiction.