Innocence is primarily presented in the characters of the littluns and Simon. As the youngest, the littluns are the most immune to the evil slowly spreading in the form of Jack's hunters. They only know to eat and play; they don't know about hunting or building shelters. therefore, they cannot be a part of the true battle brewing on the island. Although they bring the fear of "the beast" to the island society, it is an innocent fear of young children. They are simply afraid of the dark, when they should be afraid of the dark within the other boys.
Simon is the other representation of innocence. He is described in alternating imagery of light and darkness, as evidenced here:
Simon dropped the screen of leaves back into place. The slope of the bars of honey-colored sunlight decreased; they slid up the bushes, passed over the green candle-like buds, moved up toward the canopy, and darkness thickened under the trees. With the fading of the light the riotous colors died and the heat and urgency cooled away. The candle-buds stirred, their green sepals drew back a little and the white tips of the flowers rose delicately to meet the open air.
Now the sunlight had lifted clear of the open space and withdrawn from the sky. Darkness poured out, submerging the ways between the trees till they were dim and strange as the bottom of the sea. The candle-buds opened their wide white flowers glimmering under the light that pricked down from the first stars. Their scent spilled out into the air and took possession of the island.
He is the only character to truly understand their situation. He witnesses the beauty of the island, and the horror of the other boys. The most spiritual of the crash survivors, he remains the one most closely tuned with nature. He is shy, and has difficulty expressing his thoughts to the other boys. Although this isn't restricted to the first 5 chapters, it is this hesitation that causes his murder, & the description of his body being carried away fully symbolizes his innocent nature:
The line of his cheeks silvered and the turn of his shoulder became sculptured marble. The strange attendant creatures, with their fiery eyes and trailing vapors, busied themselves round his head. The body lifted a fraction of an inch from the sand and a bubble of air escaped from the mouth with a wet plop. Then it turned gently in the water.
Some have suggested that this is Christ imagery as well, which would be the ultimate symbol of innocence. Whether or not you agree with that, there's definitely a sense of Simon becoming more than a dead body. He is lit up by the "strange attendant" creatures, implying that they recognize he should be taken care of in some way. Also, the movement in the water may suggest a transcendence of death.