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.