This a picture our then 7 year old son drew of New York City on September 17, 2001.  To the left of the picture is his drawing of the Statue of Liberty which he wants to go visit (that's him in the torch).  To the right are in his words "the buildings falling down," "the airplane's hitting the building," "the pilot parachuting out of the plane" and "a man running to get out from under the building."

We really didn't think he registered that much of what was going on, but the above is evidence that at 7 he was very aware of the tragic events depicted on television.