Marge: (looking out the window to the front lawn) There's just something about flying a kite at night that's so unwholesome.
Bart: (turns head to the window. In monotone with glazed, glaring eyes and an eerie smile) Hello, mother dear.
— The Simpsons
They look sweet, innocent, even angelic, but there's something not quite right about them. They're too calm, too knowing. They aren't really children any longer, not at heart.
Children should be innocent and in need of adult protection. By inverting this, the trope arouses deep-rooted fears. The Creepy Child might not be physically dangerous, but their profound unnaturalness is just as chilling.
Creepy children are frequently female, and often Emotionless Girls. They can be, among other things, a Robot Girl, an Oracular Urchin, a changeling, a Waif Prophet, or Evil. As prophets, they emanate otherworldliness; as heroes, they may elicit distrust and contempt from the rest of The Team sans the one female who wants to be surrogate mother, and as villains they remain cute even while the bodies pile up around them. They usually have an Ironic Nursery Tune theme.
May be holding a Creepy Doll. A girl who is a Woman in White is usually the Creepy Child. If a Creepy Child is shown drawing, it will usually be a Nightmare Fuel Coloring Book.
Compare and contrast Psychopathic Manchild and the usually much more proactive and physically dangerous Enfant Terrible.
See also Undead Child and Ambiguous Innocence. Also compare Creepy Cute.