Not only do you need to educate all your children about what to do if they see your gun around your home, but what to do if they see one around a friend's home, and what to do if the see someone else find one.
It's not enough to teach them to never to touch a gun without permission and/or guidance of an adult, but also to quicky find a grownup if they see someone else find one.
My daughter was 6 when I first started carrying. From the first time she saw one of my guns, continuing on today whenever we go out shooting, I have her tell me the rules of gun safety and what she'd do if she found a gun somewhere (whether alone or with friends).
No matter how many safeties, locks, locked-boxes, closets etc. you may use to keep a useable gun out of your kids' hands, there is always a chance (no matter how small a chance) that they will still find themselves in possession of one. If you teach them right, and raise them right, there still won't be a problem.
Whether its guns, alchol, cigarettes, drugs or whatever, I don't rely on my kids not being ABLE to do the wrong thing, but on having raised them properly so they KNOW what the wrong thing is, and being responsible enough to not WANT to do it.
I'm proud to say that it's worked so far. Kids aren't perfect, but are probably better people than I am.