Good question but I think you already answered it. It's your right, and say so in as polite of a way as possible. You might also explain that you don't want to tell someone how to raise thier children, but introducing young-ones to fire-arms the right way, meaning saftey safety safety, is always a good thing because it gives children the means to handle a situation with a firearm safely. Also mention that you're not trying to hurt any body, there's nothing of malice
Really the MOST disarming thing you can do to an Anti is to be level-headed and polite, not matter what. If they're screaming in your face, calmly say good day and walk away. If they want to talk about it, do so, but don't be accusatory (they most certainly will), just state the facts as you know them. Also, know the facts heh.
Politeness and a smile will get you miles with people. Not all, but most.