Make your sentences, paragraphs and chapters fairly short. Use language and words appropriate to the age group you are aiming your book at. Plan your plot carefully and develop your main character particularly well. Your main character must be sympathetic, and the reader must identify with her.
Maintain the pace throughout the story. Let the reader know something the main character does not. Sometimes, suspense writers make a seemingly friendly character turn into a villain. Use cliffhangers effectively throughout the book, particularly at the end of chapters. According to author John Grisham, you can put your main character in physical danger in a children's suspense book. However, you should eventually save him.
Use the setting of the story to create atmosphere. You could use obvious metaphors like the gnarled branches of a tree, to represent fear and a creepy atmosphere. You could also use the weather to add to the fear and discomfort of the main characters.
Reveal your plot gradually, saving some of the main twists and turns until the end of the book. Finish the story on a with the resolution of the problem and a note of hope for the future, so the child feels he has been on a scary journey and has arrived safely home.