The “littlun’s" represent various stages of innocence. Innocence has multiple meanings which most certainly can apply to the "littluns," such as freedom from sin or moral wrong; simplicity; absence of guile; lack of knowledge; harmless, and even chastity.
Among them are:
Percival Wemys Madison: darker skinned, cries a lot. He sees the “beast” in the sea and breaks down from fright. Eventually he can’t recall his name.
The Boy with mulberry birthmark who disappears very early in the novel. The inference we make of his disappearance is that he is burned to death after the boys start the first fire, which they let go out of control.
Johnny: six years old, sturdy and fair, already belligerent.
Wilfred, Stanley, Bill, Harold, Phil are others of the uncounted group.
The innocence of these boys cannot go unnoticed throughout the novel. And it truly manifests itself in the fact that they are so frigthened by the unknown, which to them is the "Beast," a creature of their innocent minds.