Lot of issues here.
Most kids know the difference between "pretend" and "real." This despite what the child-development experts tell us. If you give them toy guns, they know the toys are only toys. But whether you give toy guns to your kids or not, you absolutely must explain to your child what to do if they come across one and aren't absolutely, 100% certain it is a toy gun.
Apart from that basic safety issue above (which applies whether your kids have toy guns or not), it ain't the kids you need to worry about, it's the grownups.
The reason toy cars are okay where toy guns scare people is obvious when you think of it: toy cars are not life size and no one has ever mistaken a toy car for a real one, except maybe in one of those terrible Japanese horror flicks so beloved during my childhood.
The line between "toy gun" and "real gun" has been badly blurred by the advent of very, very realistic airsoft guns. I defy anyone to so much as glance at one of the toy revolvers from the old days and mistake it for a working revolver. But most airsoft guns are designed to very effectively mimic real weapons, and do it so well that it's fairly common online for discussion threads to go for days debating whether a given picture is of an airsoft gun or an actual firearm.
Even non-functioning toy guns are available in very realistic shapes (though it's hard to find 'em in realistic sizes). Worse than that, even the fake-looking ones look real to people who didn't themselves grow up around guns, who haven't held a toy gun since they were in grade school (if then), and who have never held a real firearm in their entire lives.
People have gotten stupider. It's not the fault of any individual person, and individually most people are still bright enough to keep the drool off their own chins, but the culture has changed so much that people don't expect to see a 6-year-old playing with a cap gun in his front yard anymore. At least, there are enough people that don't expect to see it that someone is likely to call the authorities and scare the crud out of the kid when Officer Unfriendly shows up to find out what's going on.
For all the above reasons, when my kids were old enough to play with toy guns, we greatly preferred to give them plant misters instead of squirt guns (plant misters shoot further, hold more, and break less often anyway). They made plenty of Lego guns & stick guns & such; again, we figured none of those could cause even the most vaporish nanny type to call the cops in a panic.
We didn't let our kids have "real looking" toy guns until they were old enough to buy their own, at which time we explained that they wouldn't be allowed to play with toy guns anywhere visible from the road, or at anyone else's house unless their parents knew & approved both of the guns & of our rule about staying away from public view. We encouraged them to shoot at "invisible" bad guys rather than at each other.
Oh, for a very long time we didn't allow toy guns that really fired stuff (Nerf balls, darts, etc). But that wasn't really a "kids & guns" issue. We just got tired of settling fights about 'em. How do you figure out who is really a noncombatant, or who was just trying to get his brother in trouble? We gave up...
When we purchased our oldest son's first little BB gun, we also purchased a $10 "document safe" from WalMart, one of those cheesy metal boxes that comes with a couple of keys. He learned to lock it up when it wasn't being used and got a kick out of doing so. With the BB guns, we began practicing obeying the Four Rules at all times; I always thought of BB guns as being like the training wheels on a bike.
And every time I brought a new real firearm into the house, I made a point of gathering the kids together and introducing it to them. "This one's not a toy, kids, it fires .22 ammunition. If you see it lying around, what do you do?" They'd tell me the rules, and I'd go lock the gun away.
Probably more issues to consider, but those are the chief ones that come to mind right now.
pax