This is a hard question, because war typically is not a children's topic. In light of this, some sensitivity will be necessary. Let me give you a few examples.
First, you might want to talk about the topic of bullying. You can say that Germany was acting like a bully. They were trying to conquer people and nations unjustly. You can use the example of the Jews and nations like Poland.
Second, in connection to the first point, you can speak about the need to stand up to bullies. You can mention that many nations acted together to stand up to this bully. And as a last resort, these nations (the allies) had to go to war. However, this was a last resort.
Third, you can also talk about the topic of justice and injustice. You can write the the allies were just in their declaration of war. In other words, what was right was on their side.
Finally, you can talk about the peace that ensued in the end. Moreover, you can talk about the peaceful relationships that now exists.