The Boy in the Striped Pyjamas

My 12-year-old daughter recommended this book about the Holocaust, after reading it at school. It's based on a genuine friendship between a German boy whose father is a Nazi commander, and a Jewish boy who lives on the other side of the fence (although never specifically named, the fence belongs to the concentration camp at Auschwitz). 

It's a thin book - you'll fly through it in a couple of sittings. Even though it's very sad, I think it's an appropriate introduction to the Holocaust for children, because it's appropriately vague on some of the details, so the reader fills the gaps based on their knowledge of this horrific period of history.