Personally, I'd think sharks, even animated ones, were scarier than anything in the Saga. I saw Star Wars when I was 6, and could've easily handled it a year or two prior to that. I'd probably say 4 myself (not that I've got kids, mind you).
I'd think that the lopping off of Jango's head, and slicing Maul in half might be a little more traumatic than stuff in the OT. In the OT, Luke's chopped off hand could be scary, and other things like the Tusken Raider attack and Trash Compactor scene possibly. But I think the OT's a pretty safe place for a youngin'. 5 for sure, if not 4.
So hurry up with the movie, and stop denying the poor kid!