Well I've done it.
I let my 4-year old watch Star Wars (part IV if you want to get technical). Then I let him watch The Empire Strikes Back, just for good measure.
This despite the fact that Revenge of the Sith is undoubtedly too violent for him to see. Add to that recent news items criticizing the marketing of the movie, rated PG-13, some of which is aimed at the preschool set (Darth Tater, anyone?).
Now, in the marketers' defense, I must say that looking at the big picture, it's not about getting 4, 5, and 6-year olds to want to go see the movie, necessarily. It's about them wanting the 20th anniversary DVD box set for their 16th birthdays. Residual income, that's where the real money's at.