At the time it happened, youngest DS was 3 and not paying much attention. Eldest DS though was 8 and plenty aware. It's never easy as a parent to know how much to explain and what is wise to hold back on until a later time.
I think at this point truth might be the best policy...oh, not necessarily all the details mind you, however, the truth of the matter is pretty simple at this point and doesn't need to be related in a complex manner. Keep it simple and truthful, something like....
"There was a group of people over seas that decided it wanted to make a point in a very heartless and violent way. They tricked people, stole planes while in the air, and crashed them into some very important buildings. A lot of people died that day. Our country was very sad, shocked, and it changed the way we as a country view our responsibility to protect ourselves from such a threat. Since that time the government has put a lot of security measures in place to help ensure such an event doesn't happen again."
If you kids inquire further, explain about airport security, Air Marshals, hunting down such people no matter where in the world they are, etc.
It's okay for kids to know that not everyone in the world is safe, it's important in fact. It's also important to let them know that many things are being done to help ensure such an event doesn't happen again.
