I can certainly understand why some parents don't allow toy guns. But for me personally, I don't see an issue with it. I grew up with water pistols and other toy guns. I heard stories when I was young of Dad shooting guns, at least once at someone (he works in corrective services). Now as an adult I would love to go to a shooting range, but never want to actually own a gun. I think I've turned out pretty much ok despite my playing with 'guns' as a kid. My 3 yr old niece hasn't got any toy guns but will often 'make' a gun out of some other toy or object, even if it's just a pencil, and say 'bang bang'. So I think it's fairly normal for kids to play with toy guns, even if their 'gun' is something else entirely, like your kids with their textas.

As far as where to draw the line between ok and not ok, I'm not sure. I know my sister is fine with my niece pretending she has a gun and saying "bang bang", but there has been times where she's said "bang bang, you're dead" and my sister has told ehr that's not ok cause that's hurting someone.

I think at the end of the day, you really just have to go with what feels right for you and what you are ok with teaching/telling your kids. I don't think there's really a right or wrong answer here.