I saw the OT at age 5 (original cuts), then Phantom Menace when I was 9.

When I was 11 I rediscovered the genius of the OT in a more mature way (specifically ESB which I was completely floored by in terms of how good it was-my brother always wanted to watch ROTJ growing up so I hardly remembered ESB).

Then I saw Ep. 2 in 2002 and Ep. 3 in 2005.

I think it is good to show the prequels to kids, but they need a period of time (for me 4 years was enough) where it's just the OT and they can get that universe/framework firmly established.

Because to this day, I enjoy watching Phantom Menace and AOTC. Phantom Menace because it seemed so sweet and innocent and there is such nostalgia remembering all the toys and seeing the Darth Maul saber battle in the theater. AOTC because of nostalgia again, but also because I was 12 and I thought Natalie Portman looked a LOT better in that one than she did in Ep. 1.

But by the time Ep. 3 rolled around I was 15 so I don't have any childhood nostalgia attached to it- I see more flaws than anything.

So I think if your kids are to love the entire Star Wars saga they should be introduced to the original version OT as early as possible, then show them the prequels at age 9 or 10. When they grow up they will remember the prequels for nostalgic reasons, but they will almost certainly rediscover the greatness of the OT as well.