1. The Shining twins: Put two teenage boys in the hallway of the Overlook, or a couple of adult women, and it's no big deal. Put a couple of little girls in pretty dresses in the hallway, and it'll have you sleeping with the lights on.
2. The boy from the George C. Scott film, The Changeling
3. The boy from The Grudge (or the Japanese original, Ju-On)
4. Gage Creed from Pet Semetary (my Achilles tendon hurts just thinking about it)
5. The girl from The Ring (those Japanese sure know their creepy kids!)
6. The girl from Mario Bava's Kill, Baby Kill (Operazione paura)
7. Damien from The Omen (1976)
8. The boy with the mask in The Orphanage (El Orfanato 2007)
9. Michael Myers (Halloween, 1978): the opening scene where little Mike kills his sister
10. The Three Men and a Baby ghost: I know they say it's a cardboard cut out, but I don't care. When the urban myth got out that there was a real ghost captured in a lousy Steve Guttenberg film, it was a horrific rush. It was just grainy and sinister enough to be the real deal!
I'm sure there's a bunch I'm leaving out, but these stuck out the most. If you can think of any others that should be on the list, let me know.