Some of the littluns are crying in sympathy for the one who started it. What is it?

There's two different reasons the littluns begin crying in this chapter. In Chapter 5, Ralph calls a meeting. It's different than the others, because it's called so late, & it's really the last meeting they'll be able to have as a cohesive group. During the assembly, talk of the beast begins. The first boy to talk about it, Phil, causes the others to laugh in nervousness. All attempts to rationally talk about the beast are shot down, & Jack calls forth another littlun who claims to have seen it.

This is Percival Wemys Madison, the boy who recited his name, address, and telephone number at the beginning of the book. He tries to this again in Chapter 5, but he can't get any further than his address. The loss of his telephone number becomes a reminder of everything he's lost (although he cannot articulate this) & he breaks down. Jack yells at his to stop, but he cannot. The other boys follow suit:

A spring had been tapped, far beyond the reach of authority or even physical intimidation. The crying went on, breath after breath, and seemed to sustain him upright as if he were nailed to it.

“Shut up! Shut up!”

For now the littluns were no longer silent. They were reminded of their personal sorrows; and perhaps felt themselves to share in a sorrow that was universal. They began to cry in sympathy, two of them almost as loud as Percival.

So Percival reminds everyone of the society they have left behind: not just parents, family, friends, but all the comforts and security such a society offers.