The littluns are the youngest boys on the island. They are referred to in the collective sense which suggests that their individual identities are not of importance. One of the biggest ways that the littluns contribute to the story is that they help develop the other characters. For example, Jack has little concern for the littluns, and even suggests that a littlun be used as the pig in their next dance reenacting a successful hunt. His lack of concern for their safety and laughter at the thought of "hunting" one adds to his savage characterization. Piggy takes care of the littluns and is the only one that tries to record their names so they can be looked after properly. This helps develop Piggy's character as nurturing and maternal.
In Lord of the Flies, the littluns are the smaller boys on the island. As stated in the above response, there does not seem to be a definitive age at which the boys are considered either littluns or biguns. The littluns are characterized as the boys who need to be taken care of by the older boys who assume positions of leadership on the island. Most of the littluns are not named; however, there are some who are prominent. For example, Percival is one of the named littluns--he is the smallest boy on the island and he is symbolic of the irrational fear that the boys have of things coming to "get" them. Percival is often heard crying, and he sets the other littluns on edge. Another prominent littlun is the boy with the mulberry scar--although he remains unnamed, he is symbolic of the boys' downfall because he goes missing early in the story.