In the children's book Hana's Suitcase by Karen Levine, what are the characteristics of the book that make this book challenging and/or controversial for children?

Karen Levine's Hana's Suitcase is a nonfictional account of the life of Hana Brady (Hana Bradyova), a young Jewish girl who was killed in Auschwitz in 1944 while interred at the camp. Fumiko Ishioka, a Japanese Holocaust educator, displayed Hana's suitcase from her time at Auschwitz at an exhibit in 2000, after which Ishioka traveled to Canada to meet Hana's surviving brother and learn more about her life. Ishioka's journey and Hana's story sparked Karen Levine to produce a documentary about Hana's life before and during the Holocaust, which she eventually turned into this novel.
The topic of the Holocaust can be sensitive for children, and a story for children about a young person separated from her family and ultimately killed is controversial. The fact that the book is written for children as an audience makes it a provocative topic. It can be challenging for children to imagine a world in which a child might be persecuted by the government, forced to live in a work camp, and killed. It is precisely because of this horrible reality for Hana that Levine created the documentary and wrote this book. Hana's story is awful, but it is real. The best way to prevent future tragedies is by not forgetting the tragedies of our past.

In Hana's Suitcase, the biography of Hana Brady is interwoven with details surrounding the creation of Tokyo's modern-day Holocaust Education Center. Both the book and the center share a goal of humanizing the Holocaust to those learning about the event over seventy years later and creating an appreciation for diversity to ensure that such history is never repeated. Hana's tale concludes with her execution in a Nazi concentration camp at age thirteen, and her suitcase is one of the artifacts displayed in the center with the goal of conveying a personal impact of the Holocaust to children today.
Since children are often shielded from unpleasant topics at a young age, much of the heavy subject matter presented in this book could be considered controversial. While Hana's early life is shown to be spirited and happy, this changes when German forces invade Czechoslovakia and inflict misery upon Jewish inhabitants. The topics of World War II, anti-Semitism, and Nazism may be considered controversial by those who do not wish children to be privy to heavy matters. Specific traumatic events in the book include Hana's parents' arrest and removal by Nazi soldiers, Hana and her brother George's sentence to the Theresienstadt concentration camp, and Hana's death in the gas chambers at Auschwitz. Hana's Suitcase is undoubtedly more emotionally challenging than many children's books because it accurately conveys a vivid picture of the genocide, abuse, and terror inflicted by Hitler's regime.