I spend a lot of time teaching Boy Scouts and Venturing Scouts. I usually start with a gun familiarization exercise, letting the Scouts handle various types of actions to learn the different ways to clear, enable safeties, etc. I usually have about 8-12 kids, several gun types, and LOTS of adult supervision.
As part of this, I always drum into their heads that you ALWAYS check the chamber when handed a gun, even if you just witnessed someone else clear the chamber. We then do a "handoff" exercise, where they practice how to safely hand a gun to someone else: finger off trigger, muzzle in safe direction, check if unloaded, and handoff, then the recieving person immediately repeats the same steps.
These sessions quickly become a cacophony of clicks and clacks as the kids practice handing the guns off to each other. Invariably, someone will giggle at the absurd sound of several guns being cycled over and over, and the mood lightens and they have a good time with it. And that is how Scouts learn best - by learning in a fun environment and through repeated practice!