I talked to my niece and nephew when they where about 4 or 5 YO. Mostly to quell any mounting curiosity. My wife was sitting them one day when I got home from the range, so while I was cleaning and putting away I talked to them about it. Supervised, I let them hold and handle the rifles and pistols I had out at that time. I let them know that these are NOT TOYS, and ANYTIME they wanted to see them, they could let me know, and I will gladly bring them out and supervise handling the firearms. We spent at least 45min to an hour talking about things like the trigger and not placing your finger on it, where never to point the barrel of the weapon, etc. (no ammo anywhere around of course)
I think that many of the problems that kids encounter with guns is that they are treated as 'taboo' or some kind of wicked device that needs to be hidden from sight and never touched. Of course curiosity will dictate they need to 'sneak a peek' to discover what this is all about. 'Accidents' happen this way. I NEVER deny a request to view and handle the guns from the kids. Also, this is the perfect time to teach the respect and proper handling of the firearms, while you have a captive and attentive audience.
I look at it like kitchen knives. All the people I know never lock up their kitchen knives inside of safes, yet I don't know of any of their kids accidentally stabbing themselves or siblings. Generally kids don't bust open drawers and start playing with kitchen knives a lot, why, because adults have probably taught them that they are not toys, but dangerous tools for specific uses, they are not really curious about them because they see them daily, and in common use. Does these types of accidents never happen? They probably have happened, but it is most likely not the norm.
Now I am not saying to not lock up your guns.... of course you are going to secure the tools when not in use and under your close supervision. I am just saying that quelling curiosity can go along way towards safety.
All of this is IMHO, so this is just from my own experience.