What does one of the "littluns" see in the forest? Why doesn't Piggy like the boys' behaviour, later in the same chapter?

It's Percival Wemys Madison, probably one of the most important littluns in the novel for charting the boys' decline into charity. And it takes some persuading to get him into the assembly:

At last Ralph induced him to hold the shell but by then the blow of laughter had taken away the child’s voice. Piggy knelt by him, one hand on the great shell, listening and interpreting to the assembly.
“He wants to know what you’re going to do about the snake-thing.”
Ralph laughed, and the other boys laughed with him. The small boy twisted further into himself.
“Tell us about the snake-thing.”
“Now he says it was a beastie.”
“Beastie?”
“A snake-thing. Ever so big. He saw it.”
“Where?”
“In the woods.”
Either the wandering breezes or perhaps the decline of the sun allowed a little coolness to lie under the trees. The boys felt it and stirred restlessly.

Percival thinks he's seen a "beastie", a "snake thing", which Golding actually makes quite clear is just him having seen the creepers and vines hanging in the trees. It's the start of the fear about the beast.

And it's no accident that after this first mention of the beast, Jack immediately incites the boys to build a fire. And the boys all go racing off to do it:

“A fire! Make a fire!”
At once half the boys were on their feet. Jack clamored among them, the conch forgotten.
“Come on! Follow me!”
The space under the palm trees was full of noise and movement. Ralph was on his feet too, shouting for quiet, but no one heard him. All at once the crowd swayed toward the island and was gone–following Jack. Even the tiny children went and did their best among the leaves and broken   branches. Ralph was left, holding the conch, with no one but Piggy.

This sudden exodus is what provokes Piggy's disdain:

Piggy’s breathing was quite restored.
“Like kids!” he said scornfully. “Acting like a crowd of kids!”
Ralph looked at him doubtfully and laid the conch on the tree trunk.
“I bet it’s gone tea-time,” said Piggy. “What do they think they’re going to do on that mountain?”
He caressed the shell respectfully, then stopped and looked up.

Piggy can't really contemplate fun: and the lack of responsibility of the other boys - and their childish glee in their newfound task. It's one of many ways in which he's set apart from the other boys on the island.