I suppose he would probably face some harsh criticism for it today, but my dad kept his gun where I could easily access it, along with the ammunition. For some reason, he kept it unloaded, but he did teach us kids how to load and use it. Granted, we were blessed enough that he was able to literally take us out back and teach us to use it. Not once did any of the four of us grab the gun when he wasn't around, for any reason. It was a .357 magnum and I first fired it when I was about 7 (that means my siblings were 11, 10, and 6) when he first got it. Oddly, even though he grew up on a farm, he didn't wind up with a handgun until then.
Long story short - he taught us to load it and fire it, and showed us where he kept it. We all had the proper respect for him and firearms in general so it was never a problem.
Now that I have a three-year-old, I find myself trying to figure out how to introduce him to the wonderful world of firearms. I just hope that I can do as well as my father did. For now, until he is older, I keep my firearms locked and out of his reach and I do well to teach him that they are off-limits.