You could write about haunted houses or creepy forests. Kids often find those settings scary and exciting.
Writing about children's adventures in a forbidden cemetery or a witch's cottage could be interesting. Or maybe a story where a child discovers a hidden room with a terrifying secret in their own home. These kinds of themes usually capture kids' imaginations and give them that thrill of fear.
Well, mysterious creatures like monsters or ghosts can be a great theme. You can also incorporate elements like dark magic or lost treasures to add mystery and fear. Also, abandoned places like old schools or factories can create a spooky atmosphere.