Since you are focusing primarily on the safe use of firearms, then concentrating on the 4 rules is your best plan, as you said.
I would also explain that just because a gun has a mechanical safety, it does not mean that you can be lax with the rules. Safeties are mechanical devices which can break, so you can use them, but it is probably wise to handle the weapon as if such devices are not present. Also, I would remind everyone that many negligent discharges are preceded by these 3 words: "It isn't loaded."
I would also explain that hiding guns from Children (I'm referring to kids old enough to understand concequences) will NOT keep them safer. In fact, it is a recipee for disaster in that it introduces the "forbiden fruit syndrome", as the guns become taboo ... something irrisistable to a curious child. Once a child is old enough, they should be taught, at a minimum, safe gun handling rules, basic mechanics and shooting techniques by an instructor, knowledgeable family member or friend, etc. Start with a BB gun, airsoft or paintball gun. Heck, even a squirt or toy gun may not be a bad place to start for kids under 5.
If your kid asks to see your gun(s), allow them to with your supervision ... just be sure they are unloaded of course. After a while the mystery wears off, and the gun becomes just any other household object that should be handled with care and respect much in the same manner as a chainsaw, lawn mower or kitchen knife.
As this is purely a gun safety presentation, I'd stay away from some of the topics mentioned by other THR members (though they are certainly good topics - they just don't fit into the scope of your presentation). Keep it simple and a-political and you may just when a convert or two.
I suppose you could debunk the Kellerman Study for the umteen-thousandth time. There is also the myth that a criminal will just take your gun and use it against you. First, if that were true, then we would have the ultimate solution to violent crime: Arm criminals! Then, we could take their guns and use them against them!!!
Second, such occurences are common only in Hollywood ... I saw a statistic a few years ago and can't remember where I saw it ... basically, only something like 2-5% of the time is a citizen's gun taken by a criminal and used against them. Whatever the figure, it does not occur nearly as frequently as it does in TV and movies.