If your children are going to be around guns, they need to understand the danger. Better to teach them how to be responsible and serious about it than to have a curious child try to play with a gun.

Generally, as long as the pars are teaching them with great care and making it clear to their children exactly how a gun should be used, not to mention ensuring that their child doesn't have regular access to a gun, I don't see a harm in it. One could argue that they would see the gun as a toy at that age, but then, that would be the whole point of the teaching process here. Not engaging in an active discussion of what purpose guns have and how they should be used only leaves kids with an open perception of what guns are for.

I am okay with it so long as the methods are safe and they constantly remind them that the lesson is in self-defense and that a gun should never be used just out of anger. I feel like in a way it would also take away from the ignorance surrounding weapons if lessons are taught at a young age. As whiteflame55 mentioned, just don't give the kid regular access and only allow them usage with supervision. And I mean good supervision not that half-bummed stuff a parent would do at a pool when they think the lifeguard is watching.