The Devil's Arithmetic by Jane Yolen
I came to really appreciate Jane Yolen as an adult, especially her fantasy novels. In truth, though, she's written on every possible subject for every possible age. (According to Wikipedia, over 300 books...!)
This was, I think, my first exposure to her. This is a melancholy book, another introduction to the Holocaust meant for older children (10-12.) In it, a twelve-year-old contemporary girl is embarassed by her relatives, who went through the holocaust, and is also alienated from her Jewish faith. During a celebration of Passover, she is magically transported back to Poland in the 1940s.
I remember it as being pretty affecting. It's not just about the holocaust, really, but also about respecting religion and family, especially family history.