I know there really isn't anything to support this, but I always thought the natives were geneticly engineered and grown by Vaal. If they need a replacement, it clones out another one.
But of course that isn't the writer's premise, otherwise of all of the thousands of things that Kirk could have noticed when seeing the village, first thing out of no where is " where are the children?" I find that assinine. Then the whole "biology" discussion in the hut. Ugh. Those parts really kill this for me, whether it's stones or melons the horned gorn eats (Vaal).