Well, it IS a toy gun; fairly harmless - and anyway, if there's one thing that growing up around the FF should teach you, it's that violence DOES have consequences, and that it's not a thing to be taken lightly. I don't think Reed and Sue would have much of a problem with a dinky toy gun when they know that he's seen the results of Johnny blasting things to smoldering embers and the Thing punching through walls. If nothing else, the kid has perspective.