colorful, quick games. perfect solution: sonic the hedgehog. i've played all of the 16-bits with my son and he will watch them all (he's 1.5). he's also been enthralled with sonic r, rayman, and daytona usa.
once he starts playing, though, i think tetris will be the way to go.
I'm hardly an authority, but many of my friends have spawned and, of course, they range from casual to hardcore gamers. Their kids, however, are another story -- they are all hardcore gamers, and range in age from 3 years to 15. The one trend I've noticed, and was surprised to discover, is that they seem to universally hate retro games. If its not a modern 3D game, they abhor it. So, you've got six year olds who sneer at Radiant Silvergun, faster Ms Pac Man, 2D scrollers -- but loves his Battlefield. They all seem to be like that. Its as if the video game has to be a substitution for reality -- mirroring reality insofar as it makes some sense to them: gravity, free range movement, etc. Throw them into a game system -- with rules that don't reflect reality, and it just doesn't appeal to them.
Curiously enough, I find its really easy to get these kids hooked on two of my other hobbies -- which I always considered somewhat retro: pinball and boardgames.
Unfortunately, many kids want to play what's cool or what all the other kids are playing, so it can be hard for them to appreciate old games unless you get that appreciation into them at a very young age I think, before peer pressure begins to dictate their life. That's my theory anyway
I think cartoon based games would be great for kids, as I think Racketboy mentioned in his article. Back then, it wasn't uncommon for licensed games to be good. There were many good cartoon based games and many of those cartoons may still appeal to today's kids. Any kid who has watched Justice League Unlimited and loves Batman might appreciate The Adventures of Batman and Robin on the S-NES. Even unlicensed games like Comix Zone and Earthworm Jim are kid friendly and have timeless appeal and graphics that still hold up
You know why many of those old licensed games were so good? Because back then you had REAL companies making them, unlike today. I'm talking about companies like Capcom, Konami and Sega. When did it become a rule that only no name, lousy companies looking to make a quick buck could make licensed games? We still get some good licensed games, but it is uncommon now
I recently heard that Sega will be putting out new licensed games. Hopefully, they deliver. They've been hit and miss since the Dreamcast days it seems.