Then there are Kids films. Without making value judgements between good and bad ones, examples include Winnie the Pooh, Jimmy Neutron, The Land Before Time, Curious George, and most of the Disney films since Katzenberg's departure. Most of these appeal to the developing mind exclusively, which is why they hold little interest for adults (except where adults get to chaperone little kids to see them). Lots of them are junk food for the developing mind which is why so many of them get railed on by grown-ups.
Lucas made the mistake of trying to turn what should have been a great Family film into some kind of dumbed-down Kids film with lightsaber battles. Beyond that, he severely miscast his leading role of Anakin Skywalker with one of the worst child actors since the kid who played in Son of Frankenstein.
Ep II and III were not Family films either. The violence was ratcheted up from Ep I and therefore they required a greater level of maturity from the viewer. I'd let a five year old watch the OT and Ep I. I don't think II is really appropriate for anybody under 8 and III is probably not appropriate for anybody under 10.