Towards the end of chapter 3, Simon wanders off by himself and heads into the jungle to relax in his secluded spot. On his way, the littluns see Simon walking past and instinctively beg him for help reaching the choicest fruit, which is high in the trees. The littluns are too short to reach the ripe fruit and feel comfortable petitioning Simon for help. Unlike Jack and the other biguns, Simon is a compassionate, sympathetic boy who is approachable and benevolent. The littluns recognize these positive qualities in Simon and are confident he will assist them. Simon proceeds to reach the choicest fruit and hands it to the littluns until they are satisfied.

Simon's willingness to feed the littluns contributes to his characterization as a Christ-like figure. Simon is selfless, caring, and generous. Among the boys, Simon is the only one who helps Ralph finish the shelters on the beach and puts the littluns' needs above himself. As the novel progresses, Simon continues to display his generosity and compassion by offering Ralph words of encouragement, assisting Piggy, and volunteering to travel alone through the forest. Simon also possesses hidden wisdom and recognizes that the beast is not a tangible creature but is actually the inherent wickedness present in each boy. By helping the littluns, Simon's character parallels Christ, who spoke about the importance of protecting children in the gospel of Matthew.

The answer to your question can be found toward the end of chapter 3 of this classic novel. The littluns come upon an abundance of fruit-bearing trees, and while they are immediately eager to fill their bellies with the fruit, they soon realize that they are too short to be able to reach. It is Simon who helps the littluns reach the fruit that was just out of their reach. He really goes out of his way to help them, passing down “the choicest” of the fruit that he can reach. It is worth noting that after Simon has helped Ralph build a shelter, the littluns run after him to tell him about the fruit that they have found. This tells us that Simon is a character who the littluns are willing to trust implicitly.

The fact that Simon is approachable and willing to help tells us a lot about his character. He is depicted throughout the novel—until his murder—as a boy who is innately good, even Christ-like. Helping the littluns reach the fruit is only one of the good deeds that we see Simon selflessly carrying out. He fetches Piggy’s glasses after Jack’s punch dislodges them, and he shares his meat without thinking twice, among various other good deeds.

Because of their small size, the appropriately-named littluns are having a bit of trouble reaching the choicest fruit, the good stuff situated up high in the foliage.

It's obvious that the littluns need some assistance. So they rope in Simon to help them; he's tall enough to reach the choice fruit that the youngsters so desperately want. And he duly does so, gladly passing back the fruit to the littluns in his "endless, outstretched hands."

That the littluns should've picked Simon to help them get the fruit is highly instructive, to say the least. They instinctively feel that he is a kind, decent soul who is always willing to help other people.

The same could not be said of everyone on the island. One certainly can't imagine the youngsters approaching Jack or Roger, for example. Not in a million years. The very idea of these two brutes helping someone is simply too ridiculous to contemplate.

But Simon's not like those two rough beasts. He's a nice young man with a sensitive soul whose oneness with nature indicates a fundamental empathy with the world and everything in it. He's just the right person, then, to help the littluns get their tiny, hungry paws on some nice, juicy fruit.

In Chapter Three of Lord of the Flies, the littluns discover acres of trees in bloom with both flowers and ripe fruit waiting to be picked. They are eager to consume the fruit but are too small to reach the branches, and so they beg Simon to help them. Simon agrees to do so and plucks the "choicest fruit" for them so that all of the littleuns have "double handfuls" of the fruit. Simon then walks off on his own to survey the land and the abundance that this strange island seems to provide to the stranded boys. 

This seems to suggest that Simon has an innate goodness that the other boys simply do not possess. He appreciates both the earth and humanity and is willing to help out when others would take advantage of the situation. Simon also seems to act as a steward of the natural world. 

You are referring to the end of Chapter 3. Simon, after helping Ralph build huts, goes away and walks towards the forest. Walking through "acres of fruit trees", the group of littluns who have run after him catch up with him, and get him to pick the fruit that they can't reach:

Here the littluns who had run after him caught up with him. They talked, cried out unintelligibly, lugged him toward the trees. Then, amid the roar of bees in the afternoon sunlight, Simon found for them the fruit they could not reach, pulled off the choicest from up in the foliage, passed them back down to the endless, outstretched hands. When he had satisfied them he paused and looked round. The littluns watched him inscrutably over double handfuls of ripe fruit.

It is interesting that the littluns are described in a way that diminishes their humanity: they are not able to communicate with Simon clearly and are very greedy in their desire for fruit, glutting themselves as much as the can. Notice too, what this shows about Simon. He is clearly very different from the majority of the boys. He is generous and kind, and while most of the older boys would taunt and be cruel to the littluns, he is prepared to help them.