When sept 11th rolls around this year i will be working with some really young kids (age 4-11) at a school.
If i am asked my America was attacked, i think ill just reverse the question and ask the child what he or she thinks was the cause.
I think the response will be something like "Because some people are mean to other people."
Sometimes the most simple answer to a question is the best one.
For example while i was at work the other day a yound boy about 5 years old wandered by and i started talking to him. (kids fascinate me and i talk to them every chance i get.)
He told me that his kitten had died and he then asked me why. So i asked him why he thought his kitten died. His answer was "because he was done living." Such a simple answer but so profound at the same time.
My point is that these kids probably already know why Sept 11th happened, and us telling them about racial, cultural, religious, and social differences will only make matters worse. Instead of telling these kids what to think, ask them what they think, you may just step back and think "holy crap, it really is just that simple."