Even before the boys commit many of the atrocities in The Lord of the Flies, their predicament has secured a loss of innocence among them. When their plane goes down and they are left stranded on the island, they all are placed in a situation that no young child should expect to have to encounter. The boys are used to having their parents accomodate their needs, and the ongoing war is probably quite foreign to most of them. When they have to make due with no adult supervision or instruction, they have already begun their sudden transition into adulthood whether they like it or not. The killings, beatings, torture and other activities that later occur only add to their greater loss of innocence.

Actually, all of the boys (with the exception of Simon, the Christ figure) have lost their innocence, even Piggy. After all, Piggy joined in the dance one stormy night and helped kill Simon with all the rest; Simon, who was merely bringing the boys the news that the beast was only a dead pilot.

Tracking the boys' degeneration through the novel, one can see them, little by little, slowly shedding their innocence along with their clothing, until finally, they become much like an ancient civilization that practices human sacrifice to their gods to keep them at bay (thus the plans for Ralph's head on a stick as a present for "the beast").

And the final irony? To be rescued from their little war by adults who appear on a battleship, armed to the hilt for their own war--men who have lost their own innocence and have darkness in their own hearts. There is nobody who can truly save them, then.

All you have to do to think of the answer for this is to think of all the evil things that the kids have done while on the island.  If you think about it, only Piggy and Simon really never did anything bad, and they're dead.

As to the others, think about what the hunters have done.  They have killed Piggy and Simon.  They have tried to kill Ralph.  They have attacked those three and beaten them and stolen Piggy's glasses.  They have completely lost control and become evil.

So they are no longer innocent kids.  They are people who have learned just how evil they can be.