Children are not human. Allow me to explain.
Although children, organisms in the early stage of human developement, are spawned from human entities and possess the genetic material typical of the species Homo sapiens sapiens, they do not fit the definition of humanity. According to Merriam-Webster, the first definition of humanity is : "compassionate, sympathetic, or generous behavior or disposition : the quality or state of being humane". This, by default, includes the state of being empathetic; when we consider characterstics that are said to be humane, we consider empathy. The ability to connect with others, and experience the same emotions and mental situations they do, to "place ourselves in their shoes". Children are repeatedly asked to attempt this mental excercise throughout their lives, as part of their mental fermentation and the maturing of the social systems in their brain. It doesn't work. It can't work, because children are not empathetic.
Between the ages of 0-6 years old, the human child is incapable of empathy. When a child of this age range is asked to sympathize with another organism or describe how someone else feels, it will reply with "I don't know". They lack the ability of sensing another's emotions or thinking about another's mental state, because up until then they had never deemed it necessary. Up until this age, children are entirely selfish, if not by personality, then by psychology. As they grow older, their selfishness takes on a newer, potentially disturbing light.
From the ages of 6-8, the child becomes capable of only one aspect of empathy. It can feel the emotions of other, but not put itself in another's position; therefore, it lacks the desire or ability to help end others' suffering. When a child of this age is asked to consider how someone else feels, it will reply with "I don't care". Unless, of course, the suffering of another intereferes with its own state. Because it can experience the emotions of others, but is still inherently selfish, this is the stage in the child's developement where interesting, particularly psychopathic, behaviors emerge.
In a daycare environment, one child's stomach starts hurting. This child clutches its stomach in pain, its grotesque, tear-streaked face completely changing the atmosphere of the room, its high-picthed and feeble cries for help filling the space in the building like noxious gas. The other children, sensing this one's palpable discomfort and identifying with its emotion, crowd around it to comfort it. The scene is touching. Another child stands in the corner. The liquid emotion of the hurt child seeps into this young creature's mind, it feels its own thoughts saturating with pain, severely heightening its doscomfort. It walks over to the crying child, considers it, and promptly sucker-punches it in the diaphragm. This shuts up the crying kid for good, and the other child's discomfort ceases. The source which alters its emotional state is now eliminated. It took it upon itself to get rid of the source itself, anything to stop the effect the cries were having. Because it is selfish.
Why is the creepy ghost or zombie child monster such a cliche yet effective archetype in horror fiction? Because a being resembling a human that is incapable of empathy, completely lacking the desire to help others, is terrifying. It is uncanny. So close to humanity and yet not close enough, lacking that final step that we are so proud to define ourselves with, beyond our understanding. There is something wrong with it. It is other, paranormal, even Hellish at worst, soulless by nature. It is only natural that we witness the actions of such a creature and label it demonic.
Those bastards have to be cute in order for us to put up with them. I should know, I'm a camp counselor.