What is the significance of the description of the littlun crying as the boys fall asleep?

The crying of the littlun shows just how much the beast has gripped the terrified imaginations of the boys on the island. Ever since one of the littluns claimed to have seen a snake-monster, the boys have become obsessed by the idea that there is a beast on the loose. And this is despite the fact that no one knows for certain whether or not such a creature even exists.
The beast, and what it represents, precipitates the eventual power struggle between Ralph and Jack. Ralph is deeply skeptical of the very idea of monsters. For one thing, he knows that if the other boys are gripped by fear, then that will undermine the rules-based, civilized order he wishes to see established on the island. Jack, on the other hand, welcomes the beast myth. He knows that keeping the other boys in fear will make it much easier for him to exert dominance and control. He also knows that fear of the beast will instinctively make the other boys turn to him for protection, even if they dislike him and disapprove of his savage methods.
In the adult world such a mentality among the general population often gives rise to dictatorships. All too often in history people give up their rights and liberties in order to secure the protection of a strong state. This is what is about to happen in microcosm in Lord of the Flies.