It seems that there's a cottage industry of horror films. They crank out what seems like one every month, and they all have the same freaking "creepy" kid!
I know that the people who make these movies have no imagination, but that doesn't mean that they have to make the exact same movie. If someone made a horror film without a "creepy" kid, I would award them with some ficticious award.
I suppose the first person to use it can't really be blamed, so they are excused. Now I just have to go back and find the first work of fiction that uses children as a symbol of innocence in front of a "frightening" situation.
Not really worth the effort of looking.
For the purposes of this claim, I don't include "slasher" films, (which suck even harder) as horror. I'm talking about psychological horror, which seems to need a child as a voicebox to allude to what's going on, but not actually help.