Here's why:
* Target Audience: It's primarily aimed at children, particularly those in middle grade (ages 8-12).
* Mystery Elements: The story features a missing child, a puzzling disappearance, and clues for the reader to follow along with the protagonist.
* Age-Appropriate Content: While it deals with a serious topic, the story uses language, themes, and suspense that are suitable for young readers.
* Light Tone: While there's a sense of danger and intrigue, the overall tone of the book is lighthearted and optimistic.
Additionally, it can be categorized as a contemporary fiction due to its setting in modern-day London.