At age 3/4/5 we allowed "shooting" at imaginary targets and imaginary monster/aliens only (obviously I couldn't control what he did at school, but that was the rule in our house). No pretend shooting at real or imaginary humans, no pretend shooting at real or imaginary animals, period. No toy guns at that age, at all. He could use sticks, tinkertoys, whatever other thing he wanted to put in his hand. All pretend. As he got older, we had more talks about what shooting really means, why people do it (hunting vs target vs law enforcement/military vs criminal - using our own set of values), and how permanent it is. He got a Star Wars "blaster" when he was 6, I think - didn't look anything like a real weapon, very cartoonized. Still no pointing at actual people or animals, but plenty of pretend play enacting galactic battles with imaginary foes. His play evolved over that time period as he grappled with learning about harm and death. We still won't allow toys that look like actual guns, and never will - even our summertime water cannons bear no resemblance to real guns. Now at 8-1/2, friends his age and he all consent to "cops and robbers" kind of games where they "shoot" at each other, but I only allow it if everyone is in on it and agrees, and they have to use non-gun-resembling items, or their hands as pretend. DS has said flat out that he knows it's totally pretend and they all know it's pretend because they all get up and play again, and you can't do that in real life if you get shot. We have a few foam dart guns that bear no resemblance to a real gun but hardly ever use them - and we spend as many times with them target shooting or shooting darts across the room for cats to chase as we do playing games of "dart tag" with them with each other.
As a frame of reference: I have gone target shooting probably 4 or 5 times in my life. Have gone paintballing once, and would love to try laser tag some time. I personally see a huge divide between those activities and shooting an actual gun with the intent to harm a person or animal. As my kids get older, I work with them to help them see the difference, and until they're able to see the difference, we put restrictions on them in accordance with our values.
Ooh - as for the "how", I would just redirect with "the rule", just as I did any other thing he was doing that he wasn't supposed to. "Nope, not OK - only imaginary monsters or targets" - and I would do *that* with him sometimes, to show him I got that he had the need, and was OK with those limitations. Just redirect, redirect, redirect. I wouldn't get too much into the "whys" of the rule a ton at this age every time, because it makes it more of a forbidden fruit when it's a SUPER SERIOUS THING every time, you know?
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