"I read something that the makers of the movie felt like today's primary audience (children) wouldn't understand the times that the Narnia children were living in, and they felt like they needed to set that situation up."
See, that's what bothers me about production decisions in this movie. To me, that context is almost meaningless unless Lewis (and the producers) wanted to draw a cororally between events in the film and WWII/Nazis. The film, at least the Narnia part, is timeless and doesn't necessarily require a backdrop to any one period in history...unless it was important to do so.
According to most of the people on this thread, the corallary is the life and death of Jesus Christ. so why the WWII references?
Here's my stab:
1) Exciting opening, which audiences seem to require nowadays.
2) Shows audiences and critics (who seize upon anything to thwart a movie they don't care for) that the Pevensies had a childhood at least as hard as Potter's.
3) Peter and Edmund's experience with "The Blitz" gives them the idea to dive-bomb the White Witch's army with boulder-throwing Griffons. That closes the circle with the opening. (An example where the good guys are thinking with the prospect of a surprise to secure a win. Remember, they had to go into the battle without Aslan's help.)