Demon Dentist by David Walliams. Ages 8+.
Demon Dentist (2013) is a comedy horror about Alfie, a twelve-year-old boy who is terrified of the dentist — and has never been to one, thanks to his anxious, housebound father. When the new school dentist, Miss Root, arrives in town offering free check-ups, children begin disappearing at night. Instead of losing teeth, they are waking up to find the most disgusting things under their pillows: dead slugs, live earwigs, gobstoppers made of earwax.
Alfie, with his new friend Gabz, investigates and discovers the truth: Miss Root is a demon who needs children's teeth for a dark purpose. The climax takes place in Miss Root's surgery, where Alfie must face his greatest fear — and save the town's children — all at the same time.