This is another one of those kids / Star Wars posts, so you've been warned if you don't give a damn about such things.
So, I have shown my oldest (just under 4) bits and pieces of the original trilogy, but he has not watched an entire movie. Outside of that, he's watched some of the droids (LEGO) material but no spoilers past the Clone Wars from that show yet. Based on a recommendation here, we have watched the Clone Wars movie and have started in on the Clone Wars TV show. I think my decision now is going to be to launch directly into ROTS for his first full SW movie after CW. From his perspective, he knows Darth Vader through toys, coloring books, and a few select OT scenes, but he has absolutely not made the connection to Anakin.
Having now watched a decent clip of CW myself, it really changes the perspective of Order 66. It was a bit hollow in the movies to see clones (stromtroopers in my head) turn on the Jedi, but I don't think my kid will feel the same way when he sees it. With Clone Wars, I think my kid will have enough of the back story and really like Anakin and maybe even the clone troops... and, then, it will all be ripped away with a massive heel turn. This should be a pretty fun little journey for the two of us to share. Now, how will I explain the jobber-level stormtroopers in Episode IV?