Create a protagonist kids can relate to. This character should be someone in the same age group as the children you're writing for, with characteristics any kid can identify with. This lead character will work through the story and bring your readers into it as well.
Create an antagonist kids can relate to and find frightening. Avoid adult-themed antagonists like demons and serial killers. Instead, stick with ghosts in haunted houses, haunted dolls, scary teachers, and things in dark closets.
Focus on the "Boo!" factor. Write scenes that build tension and produce end results that make your readers either jump or sigh with relief. Do not use blood and guts to elicit fear in your young readers.
Let the good guys win. It isn't always that way in real life, or even in adult fiction, but for kids, fear should be defeated. Not only should it be defeated, but it should be defeated by the protagonist -- a peer. It's OK to scare kids as long as you show them that fear can be overcome