It hasn't come up with DS3. I think the first time I had an in-depth discussion on the events of 9/11 with DS7 was when he was four, and he came in the room when a documentary was on. I let him watch with me, and he saw quite a bit, including the planes hitting the buildings, people falling from the World Trade Center, etc. We discussed what had happened, who had caused the events and why, and the nature of terrorism in general.
In my experience, children are usually as curious about topics related to death as they are about all things. I don't see a reason to shy away from a discussion of terrorism with a child who's interested, as long as it doesn't seem to be doing harm.
Here in the United States we seem to have a general aversion to discussing death, but in my personal opinion that's unhealthy. It's better for a child to have a natural introduction to thinking about death and dying, so that it's not such a shocking, terrible event when it occurs.
