Just list every horror movie you can think of with evil kids trying to kill people. Sounds simple yet interesting, right?
Possible Spoilers
Children of the Corn series (an entire cult of kids trying to kill people)
Village of the Damned (a bunch of strange unhuman kids with no emotions who can read and control minds)
The Exorcist (The little girl Regan being possessed by the devil)
The Omen (The little boy Damien who turns out to be the anti-christ)
Ju-On and The Grudge movies (the ghost of a little Japanese boy named Toshio who kills people along with his ghostly mother Kayako)
The Ring (The ghost of a little girl named Samara who kills people through an evil video tape)
Night of the Living Dead (the Coopers' daughter becoming a zombie and eating her parents)
Orphan (The adopted Russian girl Esther going around scheming and killing people... even though she technically isn't an actual kid)
That's all I can think of on the top of my head. I'm sure you guys have seen so many horror movies featuring evil kids that you can list many that I have never even heard of.