Why are children afraid of the dark? Why are children afraid of the dark, in particular more afraid than adults? Is it an evolutionary mechanism? What causes this?
Human's fear of the dark comes from our evolutionary past. Scientists believe it is genetically encoded in our DNA to be afraid of the dark due to the attacks of predators on our ancestors mostly occurring at night. Our ancestors who were afraid of the dark survived predator attacks and thus lived, transferring that trait to us.

Fear is normal and usually occurs when we try something new, something unknown. Children, more than adults, are always experiencing new things so they have more fears which can get to them especially at night.

Fear of the dark is highest among children between 4 and 6 years old, and begins to decline at around 9 years old. Sigmund Freud believed that the fear of darkness is linked to separation anxiety from the absence of our mothers. But psychologists believe it comes from children's highly active imagination.

Fear of the dark forms at the age when children are able to use imagination but not distinguish fantasy from reality. Their heightened imagination allows young children to make a monster out of a shadow. Plus there are not many distractions in a child's mind compared to adults. Most of this fear in children comes from watching TV and reading books. The sounds and images on TV are too stimulating to their young brains and the images in a book might be misinterpreted and provoke an already active imagination.

So the general fear of the dark is natural and evolutionarily derived from our ancestors. But children are more afraid of the dark than adults because they have a more active imagination, they're at an age where they can't distinguish fantasy from reality, and they don't have as many distractions in their head to occupy themselves with so their imagination runs wild.

For more information, see the following links:
http://www.lifehack.org/articles/lifestyle/why-are-we-afraid-of-the-dark.html
http://www.webmd.com/parenting/features/fear-of-the-dark