Photographers understand that faces can change significantly depending on how they are lit. Illustrators also know this. Faces obscured are ominous. Below are examples of ominous faces making use of shadow.
Obscuring the eyes is an effective way of creating horror. In the illustration below, the deep-set eyes are entirely in shadow, or perhaps the eyes are not even there.
In children’s books, shadows aren’t utilised as often, apart from grounding shadows an unobtrusive indications of light-source. That’s because most picture books aren’t meant to be scary. However, the picture books of Chris Van Allsburg are a notable exception. Starting out as a sculptor, Van Allsburg makes heavy use of shadows, to the point where his shadows carry meaning.