Welcome to OT.com, gpforhire. Always nice to see another PT unbeliever around here. I mean, er... What prequels?!?! ;) (<-wink)
I have kids around the age of yours, and we all did SW costumes last year for Halloween.
7.5B - Vader
5.5G - Leia
4.0G - R2
1.5B - Yoda
(Mom and Dad were Hanna Solo and Dagobah Luke (with baby yoda))
So my kids are all relatively SW savvy. I gave my oldest the Droids cartoon and these oversized C-3PO and R2-D2 toys on his 2nd or 3rd Christmas. But he didn't really watch the movies until a little later. We watched Adywan's SW:Revisited as a family about 18 months ago (right before or right after the our youngest was born, I guess) and I felt like that was the first time my oldest was really paying attention. My kids generally like to turn a movie on and then turn their backs on it and play with toys... so I'm never sure how well they're following any given movie... I'm not sure how well any of the younger kids were paying attention, so maybe they'll get another kind of "first real time" later.
The decision comes down to: putting it right into their DNA from birth VS allowing them to discover it in one genuine "first" time. I leaned towards the former, and I only slightly regret that my kids will never get to truly experience:
SPOILER:
No, Luke. I am your father!
END SPOILER.
But the chances of that remaining unspoiled (in no small part thanks to yours truly) by the time they are 9 or 10 are pretty slim.
P.S. Let me know if you need to borrow my dremel to remove Galactic Heroes Yoda's lightsabre.