Why does Golding describe the savages as "little boys on the beach" and Jack in particular as a "little boy?"
In Golding's novel, no adult appears until the last three pages. The boys have been stranded on the island without adult supervision for months. This situation parallels that of the boys in The Coral Island, an 1857 novel whose premise Golding specifically set out to rebut. In chapter 1, when Ralph perceives there are no adults with them, "the delight of a realized ambition overcame him," and he stands on his head with pleasure. That image is recalled in the final chapter when the adult naval officer appears and sees what the boys have done with their "realized ambition." Although as the novel proceeds, readers may be tempted to think of the boys as older than they are because of the adult-level activities they engage in—including adult-level crimes—Golding reminds readers at the end that the boys are in fact only children. If any human being could be considered "innocent," it is a child, and The Coral Island in its naivety describes the near-perfect world the innocent British boys are able to create when left to their own devices. Golding wants to portray instead "the darkness of man's heart," and so he uses children, who should be innocent, to reveal "mankind's essential illness."
That Jack is perceived through the eyes of the naval officer as "a little boy who wore the remains to an extraordinary black cap on his red hair" is startling. Readers have come to view Jack as a powerful chief who is "a terror" and rules through fear and violence. That he, merely a "little boy," has been capable of manipulation, abuse, thievery, and murder is a powerful example of human depravity. Golding takes pains in his final pages to emphasize how "little" the boys are in order to draw the parallel to and contrast with the overly optimistic view of humanity presented by R. M. Ballantyne in The Coral Island.
Golding is trying to draw the distinction between Ralph and Piggy and their desire for order and to adhere to a law-abiding system and the wild and savage nature of the boys that comes out under Jack's leadership.
One of his main goals in writing the novel was to demonstrate that British boys were no less subject to base desires or "savagery" than anyone else as the book was a response to the story "Coral Island" which suggested that other races were more likely to descend into savagery than british subjects.
So Golding wanted to emphasize their savage nature and to point out that they were not just savage, but savage little boys and that their youth in no way prevented them from murder and other violence.