This is one of the big no-nos that seem to cause so much controversy. I always smirk and roll my eyes when someone says how they do not allow their kids to play with guns ever. My brothers and I played with them, hell we even shot BB guns (with adult supervision) and we never tried to kill anyone. And never thought guns were something to play with. So, do you let your kids play with guns? *Fake guns people*
My daughter has nerf guns and that's probably all she will ever have as far as toy guns go. However she will probably shoot real guns at some point. She will be taught how to properly handle and shoot guns and that they are not toys. She is only 4 right now though so that's still a ways off.
Toy guns, yes, they also know that unless its something like a bright orange water gun, they aren't to pull the trigger when they point it at someone and they are never to point it at anyone who isn't playing along. They also are allowed to use bb guns and real guns with supervision, my four year old has one and he's fired a real gun.