I did the same exact thing with my boys, now both 8 and 9 years old. I told them about guns when they were around 4 years old, let them hold my rifles, and then left then unloaded in conspicuous places to see what happened. None of the guns were ever touched, there was no 'mystery' to them. They knew what they were and they left them alone. I told them never to handle guns, and I would try and hand them on of my handguns, to which they'd say, "we're not supposed to toucht them daddy". Damn I've got good kids. They have been taught the same thing as I'm sure your little ones have, never point a gun at anything you don't intend to kill, and never have a finger on the trigger. When my kids have friends over, the guns get locked up. I trust my kids, it's other peoples kids I worry about.
I'm not saying this works for everybody, you have to judge the competence of you children on your own.