How does Lord of the Flies prove boys need adult guidance to grow into civilised adults?
At the end of Chapter 5, the group is breaking up after Ralph's council meeting the purpose of which was to establish more rules and structure on the island. When he opens the meeting for discussion about the beastie, chaos results. Ralph believes that even the conch cannot re-establish the order he has lost.
The following conversation between Piggy and Ralph shows their desire to have adult guidance:
"Grownups know things," said Piggy. "They ain't afraid of the dark. They'd meet and have tea and discuss. Then things 'ud be all right---."
"They wouldn't set fire to the island. Or lose--"
"They'd build a ship--"
The conversation continues with the boys discussing how much better the island would be if the adults were there. Yet, the reader is well aware of the fact that adult world is in chaos as well. The adults are fighting a world war and are unable to settle their disputes.
The children clearly need to follow the lessons that the adults have taught them. Even Roger early on in the novel throws rocks at the littluns, aiming just to miss. He has been taught by his parents and teachers that hurting people is wrong. However, adults in Golding's world are not doing any better in following their own teachings than the children.
I'm not sure the need for adult supervision is the point of The Lord of the Flies. It seems to me it's more about each person's personal responsibility and moral self-discipline when there are no external laws and restrictions to hold one accountable. These boys are young, but at boarding school they have all been exposed to the world of following adult rules and modeling the behaviors of their authority figures. They do know what they should be doing. On the island, though, there are no rules. Nothing else has changed, but nearly all of them have chosen to ignore what they know to be right and do whatever they feel. We are born inherently selfish, and Golding's point is that, without external control (in terms of laws and rules and common decency), we revert to those selfish tendencies.
If Golding intended for the adult world to be the model of all things decent and good and right, a model for the younger generations to follow, it seems to me he would not have placed the adult world in the middle of a world war. What's happening on the island with the boys is simply a microcosm of what's happening with the adults on their continents and in their countries.
Throughout the novel, the biggest proponent of the boys' needing adult guidance to grow into civilized adults is Piggy. Often, Piggy questions what adults would do in given situations and makes his decisions based on this. To Piggy, the "adult-way" is rational and logical so he thinks it is best to follow this method. Lord of the Flies proves that the boys need adult guidance to grow into civilized adults because they blatantly prove that they cannot do it on their own. The boys make a series of bad decisions because they do not communicate with each other or adhere to their initial vow of unity.
That said, I do think that the novel goes beyond the literal interpretation presented of children needing adult guidance to succeed in life. The novel is an allegory for larger ideas, so from this perspective, the boys are symbols for other groups of people.