Start at the section, "It was 8:43 in the morning..."
Get permission slips, it does contain some foul language.
This footage was shot by two French guys who were tracking the progress of a probationary fireman as he became a full-fledged fire-fighter. It contains the only known footage shot INSIDE the WTC almost immediately after the attack, the only known footage of the first plane striking the WTC, and depicts in real-time what it was like to be in the first tower as the second tower fell.
If you can, devote a FULL TWO classes to it. Just play it, and bring boxes of tissues. It will horrify you all over again. Absolutely NO commentary will be necessary. You can get an easy week's worth of lessons out of it.
I bought it for my children. My son was only 18 months old when it happened, and my daughter was born two months after it happened. I remember it like it was yesterday. I want them to understand what it was like, to be able to grasp, even tangentially, the magnitude of the event. I don't want them to have that same dulled expression on their face as I did when my parents discussed the bombing of Pearl Habour.
Forget the internet. Go with a medium they know so intimately, and is the most effective - TV. Have them do supplemental research on the internet. But for the center point of your lesson plans, use the video (preferably the DVD, which will allow you to jump to the sections most relevant to you lesson plan for the day).