I had not discussed this with my very sensitive DD because I knew it would be the cause of much sadness and anxiety. They then did quite a lot about it in school last year (grade 2). I wished I had "gotten out ahead," as said above. It was a long time before she went to sleep that night.
Mr. Rogers talks about "looking for the helpers." This has been great advice for us. Look for the heroes, and draw your children's attention to them, when dealing with such a dark and sad subject.
Interestingly, DD brings up 9/11 when we are around tall buildings and skyscrapers, not when we are in airplanes.
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.
This is such a YMMV thing. I would never allow DD to see this in a million years. She would be incredibly distraught by it. I am sure you know your child, but I would not recommend that most children see such footage. Images are hard to erase once seen. (I myself have been careful to avoid the "bodies falling" images. I know it happened. I don't need to see it.)
