The US Holocaust Memorial Museum has a bunch of helpful, age-appropriate resources and guidelines for teaching about the Holocaust.

Interestingly, the age you mention is basically where their teaching resources really take off, ie Grade Six and above. Age-appropriateness is discussed at the above link. They note that Holocaust education tends to start with eyewitness accounts (like Elie Wiesel's Night or Anne Frank's diary), which have the benefit of providing relatable figures for younger students to empathize with, but also don't really provide historic context.

Links to teaching materials are here, and for whatever it's worth since the museum is in the US, there is a focus on America ("Americans and the Holocaust", "Nazism and Jim Crow"), but there are materials that cover the topic more generally, as well as discussing the history of antisemitism, the role of propaganda, and the role individuals played. Some "foundational" teaching materials are here, but there are also videos.

Finally and perhaps most relevantly to this question I'd recommend their Guidelines for Teaching About the Holocaust. Frankly these guidelines are something a lot of people, not just educators and students, should really think on. I'm going to repost the bullet points:

• Define the term “Holocaust.”
• The Holocaust was not inevitable.
• Avoid simple answers to complex questions.
• Strive for precision of language.
• Strive to balance the perspectives that inform your study of the Holocaust.
• Avoid comparisons of pain.
• Avoid romanticizing history.
• Contextualize the history.
• Translate statistics into people.
• Make responsible methodological choices.

I won't really repeat what all the bullet points mean, unless anyone has specific questions, mostly because I think the USHMM website does a better job of explaining them.

The resources provided above are really great, you could also look up two curriculums often used with students of that age one is from Yad Vashem and is called Echoes and Reflections, and the other is called Facing History and Ourselves which is particularly good for American students. As a Holocaust educator myself I would actually try reframing a lot of those “why” questions to be “how” questions. “Why” has an extremely complicated moral meaning, and ultimately you cannot find a “why” for the Holocaust because it was an expression of the absolute absence of human morality. The question of “why” in the Holocaust is both impossible to answer and can even detract from understanding because you get so bogged down in questions you cannot answer. By reframing “why” questions into “how” questions you can get a better understanding of the historical process of how antisemitism developed in Europe, how Hitler came to power, how people could be convinced killing was required of them, and how the victims and survivors experienced the Holocaust in varying ways. Hope that helps!