1) Regan in The Exorcist
"I'm telling you that 'thing' upstairs isn't my daughter." When sinister facial expressions and subtle storytelling are not enough, ain't nothing wrong with some head-spinning, green-spewing, levitating fun. Linda Blair portrayed the possessed 12 year old; first as the innocent little girl who has a game with a ouija board, then developing into, well, the devil himself. Maybe. The film may suffer from the odd moment of unintentionally humorous bad dubbing, but as a whole, it's still pretty intense. Like The Bad Seed, the real horror lies not in being a victim of a murderous child, but in being one of her parents. Regan's mother Chris (Ellen Burstyn) is stuck with the prospect of a demon for her daughter and nobody who can offer her any help or even a rational explanation. Linda Blair was nominated for a best supporting actress Oscar; unfortunately it emerged that between Mercedes McCambridge providing the demonic voice, and Eileen Dietz acting as a stunt double for the more violent and icky scenes, Linda's contribution was questionable. However, she did manage to startle Max von Sydow with her depraved lines while shooting the exorcism scenes, so all was not lost. It's not subtle, but it is one of the best horror films ever made, and Regan (and pals) created not only the scariest child in the movies, but quite possibly the scariest single character of all time. Wowser!
Honourable Mentions
Children of the Corn: The delightfully depraved Stephen King short story was made into a truly turgid movie. The kids are less frightening, more want-to-smash-them-in-the-face-annoying.
We Need To Talk About Kevin: Ezra Miller plays the title role; unfortunately he gets little screen time as the straightforward horror of the novel is avoided in favour of an arthouse flick about the reaction his mother (Tilda Swinton) has to his high school rampage.
The Shining: For shock value the twins are probably scarier than Regan, but they lack the character development and spiderwalking novelty.
Let the Right One In: Lina Leandersson (as the mysterious, super strong Eli) sure was hungry, wasn't she?
Case 39: Renée Zellweger turns crusading social worker when Lily (Jodelle Ferland) needs help. As it turns out, the little girl isn't as innocent as she appears. It's a ludicrous film, notable only for the comedic use of "Keep away from children" featured prominently on an essential box of matches...
The Good Son: We all had a friend at school who was a bad influence, didn't we? Macaulay Culkin plays the inexplicably evil Henry, who is essentially Kevin McAllister, but with a really dark side. The result is a film which is too silly to be really scary.
Lord of the Flies: What happens when children are stranded on an island with no grownups to keep order? As it turns out, they descend into savagery faster than you can say "Looting at Footlocker." The horrifying story leaves you with the million dollar question "What happens next?"