The juvenile market includes both picture books and chapter books. There aren't many scary books for children under the age of 8, so the preteen market is ripe for stories. Preteens have an expanding vocabulary, and most can appreciate some colorful storytelling mechanics. Always keep the reader's age group in mind. While having to consult a dictionary isn't the worst thing a child can do, moving too far past your audience's parameters might alienate them. Keep the writing simple, direct and transparent.
Vampires, werewolves, witches, warlocks, haunted houses, zombies -- we know these "monsters" intimately. Most kids have already seen (or read) the best tricks of the genre -- your job is to give them a unique experience. If you must have a werewolf, work out an angle that hasn't been overdone in either movies, television or books. Splice genres if necessary to come up with something uniquely yours. Even if it isn't new, it has to feel new.
Choose a focal character and stick with him. You want your readers both scared and thrilled to turn the next page. Suspense works best when readers care about what will happen to the character if he investigates the scratching noise behind that door. If they care enough, they'll eagerly anticipate your character's next move.
The class bully. The first day at school. Making friends. Losing friends. Coexisting with siblings. The food in the school cafeteria. The parental divide. Adding some of these elements to the supernatural elements of your tales will enrich them by providing some verisimilitude, as well as providing familiar territory for your readership.
Chances are whatever scared you as a kid might scare your audience too. Writing what you know usually results in strong thematic material. Delving into your own childhood fears might be exactly the fuel you need to propel your reader into a strong, gripping story.
The best tip for writing horror stories for kids (or any story for that matter) is reading widely within that genre. See what writers such as R.L. Stine, Neil Gaiman and Margaret Peterson Haddix are writing for kids. Read to know what has come before. Read to understand today's trends in juvenile horror fiction. Read because it's fun, and as a result, what you write might be scary enough to raise a hair or two.