Let us us face it: children can be evil. We have all been witness to it at school, at the mall, at Wal-Mart, at others' homes, and--God forbid you have your own children--at your own home. On average, children learn very quickly what is right and what is unacceptable. The "right" part they acknowledge and store away in the their brains for later use, but what really intrigues them is the unacceptable. Children want to know why they should not urinate in the houseplant as the dog does and if there is any circumstance in which it is acceptable. In other words, they must push the buttons we do not want them to push and speak words we do not want them to say because they need to know what boundaries, if any, limit them. As annoying as it may be, this is normal child behavior and although it may be Hell on earth to deal with, it is necessary.
I have often overheard parents talking about how wonderful it would be if their child had popped out of the womb as quiet, nice, and well behaved; but would a perfect child truly make for worry-free parents?
In the the case of the 1898 novella The Turn of the Screw, written by Henry James, or the 1961 movie The Innocents, directed by Jack Clayton, the most mysterious thing about the children is that fact that they appear perfect. The book relies on careful sentence structure and word usage to imply the idea that the children are perfect. This is noted as a reason as to why Flora and Miles are so complimentary to the governess. They are drawing the reader into a misapprehension of their characters. The duo is not perfect in either the book or movie. Miles and Flora, as portrayed by Martin Stephens and Pamela Franklin, are calculating and maniacal in their interactions with the governess, named Miss Giddens in the movie, as played by Deborah Kerr. As the reader of the book or the viewer of the movie continues, he eventually comes to the conclusion that no children are perfect--they are incapable of it--so this outward cover must be used to conceal the actual evil that lurks inside. That is what makes the story creepy. Also, the book version is very good at maintaining the uncertainty of whether or not the children actually see the ghosts. Since the reader cannot see the eyes of the children, the reader has no idea if the children's eyes are focused on the ghosts or just "weirded out" by the governess.
Actually, children have long been used in movies for that extra creepy effect. The first movie that comes to mind is The Omen, directed in 1976 by Robert Donner. Little Damien, as the anti-Christ, was responsible for the deaths of several people, including his own father and mother; but his cute sinister smile remained an impression of his face till the last scene.
I suppose that for the ultimate shock, using children for a source of evil is the best solution. It forces one to rethink the idea of their carefree, innocence, and forces a family to reassess just who it is that wears the pants of power in the family--or diaper as the case may be.