Very little is known about child psychopaths. All we see is the devastation they cause as adults. The linked article chronicles one of the first attempts to get a handle on the issue of child psychopathy, and includes several anecdotes about rather chilling children. (I'll tell you one thing: I don't envy the mothers. That's a hard row to hoe.) It's a long article but if you've got a cup of coffee and some time on your hands, it's a fascinating read.
And yes, I love the movie. Did you ever notice that the guy who plays Leroy, the dumb handyman, also does the voice in the radio broadcast about the death at the school? Caught that the last time I saw it. There are many wonderful things to be said about this movie. It's one of my favorites -- in a decidedly campy way.