Saca and TR are absolutely right. If I had to pick only one between giving the kids a good gun education/demystification, or a badass locking gizmo, I'll take the education any day.
Kids are smart, and they're paying attention when you think they aren't. If they're curious enough, they'll figure out how to open anything you keep your firearms in. The key is, as TR said, taking the kids out, teaching them firearms safety, and letting them shoot. Hell, do it a bunch of times (it's a good excuse for the wife: 'Hey baby, I have to take the kids shooting some more. Education, you know....').
Once they know what it feels like, how it works, and the destructive power of it, they'll be a lot more respectful and won't feel the 'hey, I wonder what it's like to play with that thing!' drive that they would otherwise.
The locking gizmos don't hurt. But they're the backup, not the primary.