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.)
Interestingly, this was also what brought out DD's awareness months later when she was a toddler. We had to 'talk her down' while she was a hysterical mess as a not-quite-3yo, and this was elicited by passing a very tall pulp mill stack on the freeway. So completely out of the blue-- if she hadn't been babbling about airplanes and "the building could fall down" I would never have put it together.
The upshot was that I will
never forget the look of dawning horror as we-- trucks whizzing past us at 75mph-- explained that she didn't need to be worried about ALL airplanes, or tall buildings; that the events in question were
not an accident. That look will haunt me forever, because that is when my sweet little girl learned that people really are capable of incomprehensible evil. I could see her put it together. She looked puzzled at first, and then insisted that I answer the direct question; "Someone crashed the airplane ON PURPOSE??" I explained it as mental illness of a sort. I have not regretted that.
I agree WHOLEHEARTEDLY about full disclosure to the greatest extent that our kids can handle it. The reason is that PG kids in particular are
likely to absorb more information about the events than we think that they've been exposed to. Something this awful is so vastly age-inappropriate for them to try to manage alone.