We might argue that the boys believe themselves to be in a Utopia because they are suddenly living in a world without official rules and authorities. Eventually exactly these qualities of life on the island turn it into a very difficult and dangerous place.
Kids always think they will be better off without adults. Then inevitably some kid will take on the role the adult would have played. Kids don't have the same experience or self-control as adults, so the results are usually not good. This is exactly what happens in the book.
The early Utopia turns to chaos due to power crazed greed: greed to be powerful; greed to be wild; greed to experience the Rousseauian natural, unfettered life in the wilds. [So much for Rousseau .... If he only knew ....] Their life together turned to chaos because passions were not governed and moderated; reason and rationality did not have the highest place.
Despite the tragedy that brought them to the island, there was an air of utopia during the first days of the boys' stay. Frolicking in the sun on a beautiful beach is a utopian dream for many people (I hope to retire to such a lifestyle myself), and many of the boys seemed happy with the turn of events. They were children, and having fun was foremost on their minds. The absence of adults only made the situation seem more perfect--at least at first.
I agree that the example of the Utopia in the beginning is merely an illusion. The kids think this will be a Utopia but it is not. Human nature and childhood immaturity take over quickly. Without a reasonable authority figure, chaos ensues. The idea that a group stranded on an island could create a Utopia is idealistic. Golding makes the argument that this type of Utopia is an impossibility because of the violent and dark side of human nature. I believe he selected children as his main characters because readers would find less argument with his point. Readers would believe that children would be unable to control and restrain their baser instincts.
The kids did at least think the island was a utopia. This turned to chaos because of human nature. Too many of the boys simply wanted to have fun in irresponsible ways. This led to discord between them and the ones who wanted to be "civilized" and responsible. That, in turn, led to the chaos that came as Jack's group started fighting Ralph's, eventually taking control of the island. Golding argues that irresponsible, selfish, violent behavior is in our natures. This aspect of our nature was given free rein on the island and chaos ensued.
Well, I don't actually think that there ever was an example of utopia at the beginning of this novel. Sure, the kids are stranded on this island that seems as if it was some kind of paradise, but very quickly, the darkness of human nature shows that this setting is more of a dystopia than a utopia. What Golding's central point is shown to be in this novel is that mankind is inherently evil and dark. When we are removed from the trappings of civilisation, such as a justice system and police, that overtly at least keep our inherent evil in check, we resort to tribal savagery and barbarism. This is a point that is underscored by using innocent boys as characters.