Oh, dear. TBH, although I don't like censoring reading either, I think I would try to get her to return the book. I'd be straight about it: "That's a really, really sad true story and I think you might find it very upsetting. It's a good book to read, but I think you should wait until you're quite a bit older." Even if you needed to tell her the ending in outline ("She hides from the Nazis for a long time, but in the end she is found and killed.") knowing it wouldn't be as upsetting as reading the book, getting to know her as it were, and then finding out that she dies. My DS8 isn't extremely sensitive, and he does know about the Holocaust in outline, but still I wouldn't want him reading this yet.
