As a teacher who taught Anne Frank for many years, one of the novels and topics she might be interested in is also about children, but these were the children of the transport. They came from countries where the parents said goodbye to their children for the duration of the war. One novel dealing with this is called Kindertransport. Another book/topic is the rescue of Jews by good people and the book is Number the Stars by Lois Lowry. I realize you were looking for a topic rather than a book, but these two novels might be ideas you could use.
If I remember correctly, you are from England. If so, your daughter might be interested in the lives of children in England during the war. Specifically, it might be of interest to her to learn about the children who were evacuated from cities to keep them safe during the Blitz. This link has a brief article on the subject that seems to be geared towards children of your daughter's age. I would imagine that this topic would be a good one because it would be about people her own age and perhaps (depending on where you live) even from her own area.
My own 9 year-old daughter is most interested in the Holocaust when it comes to WWII. She is highly interested in the lives of Jewish families who were trying to hide from the Nazis. That seems a little dark to me, but it is something that interests her.