What's the Name of That Book??? discussion

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The Secret of Platform 13
SOLVED: Children's/YA
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SOLVED. Middle Grade Fantasy - Takes place in England, somewhere near the sea. A young English boy befriends a girl his age (who has ogre/witch ancestry.) Spoilers ahead
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In the climax, the young girl gives up and goes to live in this cave with her ogre family because she feels like a failure. The protagonist eventually convinces her to leave with him, and there’s a happy ending. This may be a children’s book.
(Details are fuzzy, I read this when I was in grade school, so some of what I remember may be slightly inaccurate.)