Rifles like the Chipmunk and Crickets are sized for kids in most all respects. The guns actually fit them, and are of a weight that is relative to their size and what kids can handle. They make learning to shoot so much easier, than trying to teach them on guns that are to big.
Smaller, light weight revolvers are also good starters for handguns, and these days, things like the Walther P22 work well too. Their grips are more appropriate to a kids grip, and they arent overly heavy.
Personally, I thinlk handguns should wait until they have the rifles down, as they tend to be more of a challenge to shoot (even for many adults), and there are more safety issues involved.
I also think the longer you wait to teach them, the harder its going to be, and especially if you wait until they have been watching TV, movies, video games, etc, and had other "bad" influences that are counter productive to what youre trying to get across to them. We never had any troubles with our kids as they got older, but their friends (and their parents) were an issue a couple of times, and a couple of the parents were scarier than the kids.