Freezing probably would kill some, but not all of the bacteria and yeast. I know there's a way to preserve kefir grains by freezing them, so enough of the bacteria and yeast must be able to survive freezing for the grains to revive. I would imagine that the kefir itself would behave the same way with freezing.
My guess would be, it would kill a few but not a lot. I've successfully rejuvenated frozen kefir grains with no difficulty at all. I, too, have tried freezing kefir smoothie for my girls, but it was still a no go *grumble*