Explain Percival reaction and thereafter that of the "little uns" in the context of what has happened this far in the novel?

The littluns are frightened, and Ralph tries his best to assure them there are no "beasties" or monsters........ but he also admits to the young ones that he us frightened to. His reassurances don't do much to alleviate their fears.

None-the-less, Phil (a littlun) goes onto tell them about a nightmare he's had, and the beast he saw moving through the trees when he woke up. Again, Ralph shrugs it off....... and in turn, Simon admits that he'd been out walking.

Percival is the next to speak; and he recites his name, address, and phone number....... a reminder of home that causes him to break out into tears. The quote you've cited above described the source of these tears...... it's like having something build up insode of you for a very long time, but you've done the best to hold it back. The problem with that is that when you finally let the emotion have its way with you it's impossible to stop it, just like the water that springs up from far beneath the earth.