You can't lecture kids about guns and expect them to stay away from guns. Your words are not that powerful. Kids need to see how powerful guns are up-close and in-person.
Here is how I would teach gun safety. I would take them on a field trip to a gun range. At the gun range, I would have them pass around and hold various objects that are very heavy and hard to break, such as a fire extinguisher. Then I would have someone at the gun range fire a bullet right through that fire extinguisher and those other objects, and let the kids see the damage with their own eyes. This would involve having the kids hold the damaged objects.
Some things must be seen to be believed.
To make this more effective, I would give the kids a pretest with these questions:
1. How fast can bullets travel?
2. How far can bullets travel?
3. List any objects that you think would stop a bullet. (I imagine that a lot of gun tragedies are caused by kids thinking something's bulletproof when it really isn't.)