What do the twins see on their night watch? How do the other boys react and why?

The twins Samneric, the two boys on duty at the fire, awake and add kindling to the fire. Just then they spot the dead pilot at the top of the mountain and are immobilized by fear. Eventually, they scramble down the mountain to wake Ralph. Samneric claim that they saw the beast.

There was a speck above the island, a figure dropping swiftly beneath a parachute, a figure that hung with dangling limbs. The changing winds of various altitudes took the figure where they would. Then, three miles up, the wind steadied and bore it in a descending curve round the sky and swept it in a great slant across the reef and the lagoon toward the mountain. The figure fell and crumpled among the blue flowers of the mountain-side, but now there was a gentle breeze at this height too and the parachute flopped and banged and pulled. So the figure, with feet that dragged behind it, slid up the mountain.

Far beneath them, the trees of the forest sighed, then roared. The hair on their foreheads fluttered and flames blew out sideways from the fire. Fifteen yards away from them came the plopping noise of fabric blown open. Neither of the boys screamed but the grip of their arms tightened and their mouths grew peaked. For perhaps ten seconds they crouched like that while the flailing fire sent
smoke and sparks and waves of inconstant light over the top of the mountain. Then as though they had but one terrified mind between them they scrambled away over the rocks and fled.

"Ralph, wake up!"
"What's the matter?"
"We saw--"
"--the beast--"
"--plain!"
"Who are you? The twins?"
"We saw the beast--"