I'll admit, I was a bit surprised that they went for the double bird rather than the single. (I was also reading as "six-four-five" hahaha) Or not going for 132. I have definitely heard "4-off" after they learn this. But then again, as kids we use to use our pinkies as a way to flip the bird without actually doing it, so I think kids are always going to encode what they can.
Indeed, beware of playing this game with kids! Mine started shouting "645!" at each other.