The littleuns range in age, likely between five years old and nine years old but it is never really indicated how many of each age there are. They seem to be unaffected in a certain way by their situation, perhaps too young to really think hard about it they just play and try to survive as best they can.
They are described by Ralph as "hopeless," as they cannot contribute to their efforts to build shelters or keep a fire going in a meaningful way. They don't stay focused and simply need to be fed and taken care of. They seem to follow whomever appears to have the most authority and, as time goes on, gravitate towards Jack who provides meat and doesn't demand as much of them as Ralph does.
In most of the story they serve as background in a way, particularly in the beginning when they serve to highlight the fact that Roger is a budding sociopath.