It depends on the child. I watched the first one (I mean episode IV) with my five year old. She wasn't scared by any of it (at least what she saw of it before she fell asleep) and was getting quite into the story. It's different watching things with them from them watching by themselves.
Star Wars is pretty mild compared to other movies out there - even some of the ones that are aimed at kids are pretty questionable in my opinion. I'd watch it with them to make sure it wasn't too scary, but my 6 year old would probably be fine with it.