My sons are 5.5 and 3.5, and are very interested in WWII. They are pretty bright but they have normal emotional development for their age. I have let them watch things like Transformers (with a little bit of fast forwarding) but WWII is on a whole other level . . . Most WWII stuff would be way too traumatic for them.
I've told them stories about their great-grandfather flying a fighter jet in WWII, about how people didn't want to go to war because war is awful and lots of people die in wars, but that the we (the allied countries) decided we had to. They know a very little bit about Adolf Hitler ("one of the meanest people ever") and the holocaust.
My older son specifically asked me to get him a WWII movie. He is very visual and I think he wants to "see" it.
The only thing I could think of was The Sound of Music but I think they would get bored with all of the singing and dancing, and much of the dialogue would be lost on them. Really they want something with fighter jets and bombers and people like their great-grandfather being heroic. I think he will love watching Ken Burn's The War when he is older.
What about the movie I Am David? I read the book North to Freedom when I was about 10, but I don't know anything about the movie. I was thinking of getting that for them. Anything else?