Hidden is a graphic novel that does a good job introducing the idea of the Holocaust to young readers without being overtly violent. Hidden features a grandmother, Dounia, who is telling her granddaughter about her experiences during World War II. Dounia wore the star of David, was hidden by her parents in a piece of furniture with a false bottom during a raid, and is rescued and taken care of by resistance members and neighbors.
Much of the violence is implied, which would allow adults to pick and choose how much information they wanted to share with young readers. I would recommend this book for second or third graders. Older or more mature students will want to follow up with books such as Number the Stars or Milkweed.