Step off is usually 90 minutes before kick off. I would highly recommend being on the quad about 2 hours prior to kick off so your family can watch the Corps fall out from the dorms and see them form up before step off. It's quite a sight to see all of the fish wildcatting from the dorm to their formation spot.
Tell them about the canon before hand. Watching and anticipating it going off is one of the highlights and to me is much more of an experience then having the crap scared out of you!