The MPAA gave Revenge of the Sith a PG-13 rating due to sci-fi violence and some intense images, but we think they were probably erring on the safe side.
Sure, there are some pretty gnarly things that happen (like lots of dismemberment, for one), but there's a conspicuous lack of blood, guts, and gore. The ickiest thing that happens is that a lot of adorably British Jedi-in-training get slaughtered offscreen.
Hmm. Come to think of it, that's pretty traumatic.