There are two basic directions I can think of to take a children's book about a specific time period. You can either attempt to address what was happening during that time period or attempt to show what a child's life might have been like without directly addressing the issues of the time. As post two suggested, you might discuss bullying or otherwise simplify the conflict between the various countries involved in WWII. Simplifying the political and social issues of the war to a level a child coud understand is one option for a children's book. The other option is to show a day in the life of a child during this time period. A child might not necessarily know or understand all that is going on around them, but they would certainly notice the changes a war can bring. A child might talk about food rationing or a parent going off to war. A European child might have had a Jewish friend that disappears into hiding or is taken away. A children's book might simply describe a period in a child's life and show the changes that take place without really getting into why these changes occurred.
You could focus on the home front in any country. In this way, you could talk about how the war affected the lives of kids, how they received news of it, even how they participated in it through things like recycling drives and other initiatives. What was it like to come of age during the biggest and most destructive conflict in human history? A children's book about the home front might get at this question in a unique and compelling way.
Are you writing a book or looking for one? If you are writing one, I would focus on the human element. I am not sure what age group you are looking for, but in books designed for children at younger ages I would focus on the broad strokes of how children would feel during a war. You will want to look at some examples.
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.