I haven't read Number the Stars since I was 11 years old, and just as with The Light in the Forest and A Gathering of Days, my memories of the story were very far removed from what actually happens. I remembered the girls being German, not Danish, and I was sure there were scenes set in a concentration camp. Compared with the dark story I conjured up in my imagination, the real thing, though compelling, was far less upsetting.
What is great about this novel, though (aside from Lowry's writing, which I always enjoy) is the fact that it makes the horrors of the Holocaust understandable to young readers without giving them too much to process at once. Through Annemarie, an ordinary girl, readers are able to imagine how it would feel to slowly see their country changed by outside invaders, and to consider how they might be able to muster their own courage in the event that a friend's family was threatened by a plot to evict them from their rightful homes. Though there is no mention of concentration camps in the story, the concept of "relocation" opens up the discussion so that parents and teachers can begin to discuss what happened with kids and help them process the disgust and fear they will feel upon learning of these events for the first time. The story also gives readers an opportunity to reflect on the hope represented by the Danish Resistance, and to realize that many lives can indeed be saved by the actions of those who are willing to stand up for what is right.
Based on this reading, Number the Stars is probably the book I will use to introduce the Holocaust to my children when we reach that point in our homeschooling curriculum. Though I originally read it in sixth grade, it would work well for most readers in grades 4 to 8.