1 They are unpredictable
Children are very unpredictable. They have a vivid imagination that can be magnificent or terrifying.
2 The theme of lost innocence
Children are universally perceived as innocent beings who can't cause harm. It is for that reason that directors and producers use them in horror films.
3 They are everywhere
Children are found in most families. It's far more terrible to imagine that someone who is so innocent can be next to you and cause horror.
4 They are open to ideas
Children are what most people see as raw material for learning and observing. It is a lot easier to make a child do something terrible than convince an adult with it.
5 For plot twists
The use of children in horror elements isn't new. It makes a great plot twist when the cause of horror is the child in a movie.
6 The purity
Children are pure, so when they are evil, it is the idea of pure evil that is terrifying. (See Why Do Some People Like Horror Movies?)
7 It's difficult to hate them
It's difficult for most people to hate children. So, when you realize that a child is the cause of terror, you can't help but have mixed feelings that disable you from thinking clearly.
8 The desire to help them
Most people would rather help the possessed or lost child than step away from them. That always gives a movie some sort of longevity. (See A list of Survival Horror Movies)
9 Fear of the unknown
Children are unpredictable. So, it's the fear of the unknown that's haunting after discovering that a child is the cause of horror.