The Secret Of Platform 13 by Eva Ibbotson

A hidden magical island is linked to the human world by a secret portal at a London train platform that opens only rarely; when the island's royal baby is kidnapped and taken to the city, a ragged but determined rescue party crosses into London to find him. They discover the child grown up in a human household and unaware of his origins, and what follows is a warm, adventurous tale of comic mishaps, unlikely friendships, and the struggle to decide where true belonging and family lie.

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