Review the horror stories already on the market that are written to target the age group you want to reach. You'll need to know what's already on the market to avoid duplicating it, and you can use those books as a guide on what's appropriate and what goes too far.
Start with an outline. Decide who the main characters are, their ages and what horror they'll face in the book. Come up with an ending as well.
Make the scary parts of your horror story age-appropriate. Rarely does anyone die or get seriously injured in a children's horror story.
Create the horror by using something that kids and adults can relate to, like spiders. Exaggerate that fear by having your characters wake up to find the entire house filled with people-eating spiders. Using situations that kids recognize makes your horror story strike close to home.
Give the kids in your horror story the tools they need to defeat the horror and return to their normal lives. The satisfaction of solving problems is what makes a horror story satisfying to read.