Note: I asked for and was given permission to post this link.
If you are interested in suggestions on how to talk to children of any age about 9/11 or other tragedies, you may be interested in this blog post, written by my boss.
My kids are 5 and 2, so we won’t be talking to them about 9/11 this year. The older one did hear something about it in kindergarten, but apparently not enough to make him curious, which is fine with me.