Lucas needs to make three more crappier movies than the PT so we can start seeing them like we see ROTJ now. =P
Ewoks? Another Death Star? C'mon.
I think Star Wars is like sex (better than?). You need to have "The Talk" with your kids about Star Wars the same as with the birds and the bees.
Certainly show them the OT first and get that drilled into their heads for four or five years, but when they're starting to come to you with questions like "How did Darth Vader become Darth Vader?", you need to sit them down and explain the whole story of the PT and OT and the two Lucas's. Maybe they won't fully understand at first, and maybe they will, but either way, they're going to end up making their own conclusions about it when they're old enough to have access to all six movies, and it's better, in my opinion, to give them advice on "safe PT viewing" as opposed to "unsafe PT viewing."
Otherwise, what might happen is they resent you for depriving them of something and then gravitate towards that as a form of rebellion. It's all fun and games until they come home with a Jar Jar backpack. Then you'll wish that you'd have explained it to them.