I always assumed that most of the children were killed off by Ceti Alpha V's hostile environment/wildlife, and the casualties among the adults eventually led to the survivors being able to procreate only at a very slow rate.
Still...the image on your linked thread seems as if it could be powerful if it'd been played right. I agree with some of those posters that it's a bit creepy, and that the child's death might've made the movie more depressing than it already was, but somehow I think it might've made things more profound too...something about 'this is what happens when man plays God' or other such.