Most of my students were not even alive yet, but their lives are surely effected by this event. Students who are currently in the fourth grade were born in 2001 or 2002.
Here are some resources that I have found that are related to 9/11:
9/11/2001: The Day That Changed America from Scholastic
9/11 Attacks - 102 Minutes That Changed America from The History Channel
OurStory from SmithsonianThe September 11 Digital Archive
Today in History: September 11 from the Library of Congress