The Evil Children are monsters from The Cabin in the Woods novelization exclusive. They are described as a group of 19 demonic children. They look like normal children but they have demonic traits. They are a diverse group with 6 African kids and 13 white kids. Some are boys and other are girls. They were mentioned in the elevator scene.
There is no known image available for them.
They are a direct reference to the movie Children of the Corn, based of Stephen King's novel of the same name. Another reference could be from the British 2008 thriller-horror film The Children.