Both our boys got a Chipmunk (pre Crickets) when they were born, and started handling them as soon as they were able. I was shooting a lot of rifle matches back then, so I was always dry firing on the living room floor, and they each practiced with me for a short while, every time I did.
They both started shooting live ammo at 4, and by then, had shot thousands of "rounds" (snap caps) on the living room floor in practice. I never let them shoot from a bench, and we always worked on good field positions. The Chipmunks, unlike cut down adult rifles, were sized to fit them, and they had no troubles shooting them well. They both fired their first rounds standing/offhand, on a chair at an indoor range I belonged to, and both put their first rounds , and most of the rest in the black on a standard pistol target at about 10 yards.
I believe the main thing, is to start them as soon as you can, and take every opportunity to talk and go over safety, and anything else. They should be allowed to handle anything they want, any time its reasonably done, and they should be encouraged to do so. If you do this, you will have no troubles, and your kids will be safer than most adults.
We never kept toy guns from our kids, and I actually encouraged them as well, but they were never allowed to have BB guns. This was something my dad started, and I believe his theory was right. He felt we would treat them as toys, which we did later on when we were old enough to get our own (pre paint ball days). It was paintball guns for our kids, and this kid had one too.
. Even more lessons learned. Lots of fun too.