My sons are 19 months old and 3 years old, and I look forward to taking them shooting. I've got a .22 bolt action rifle (Sears Roebuck) that I got from my dad, and he got it from his dad. Sadly, it has a blocked barrel, but here's hoping a gunsmith will be able to do some magic.
Barring that, I have my good ol' pre-ban Ruger 10-22 they can fire without getting knocked backwards. I'd rather they learn on the bolt-action so they're forced to spend a little more time between shots getting reset. Seems like 50 rounds through a bolt-action is better for developing skills than 50 rounds through a semi-auto, since you have to set up 50 times instead of twice when you run through your 25 round banana clip.
Of course, when they're older, they can fire the big guns. "Marcus, you'd better believe me when I tell you to put that Mossberg all the way against your shoulder, or else you're gonna....... well, I told ya, didn't I? Clear the gun, head over to mom. She brought the icepack just in case."
Still haven't bought my wife her pistol yet, we haven't been shooting in a few months. We're still zeroing in on the Beretta 9mm or the Walther P22, but we've gotta go test some more out.