You can find the answer to the first part of this question right at the beginning of the chapter. It even uses the exact words "sign came down from the world of grownups." The sign is an explosion and a "corkscrew trail" of bright light -- this is clearly from an airplane getting shot down.
What scares Sam and Eric has to do with this battle. The pilot of the plane has parachuted out of his airplane. He lands near them and his parachute sort of ties him in a sitting position. When Sam and Eric see this, it scares them.
In the book "The Lord of the Flies" the part of the plane crashing down is the sign that fell. There had been some air battle going on in a distance. The book states that the night sky was filled with more than stars. This is also foreshadowing for the later parts to come when the island becomes even more scarred at the hands of mankind. The pilot has come down from the plane and later will be the beast that the children fear so badly.
Sam and Eric are scared because they are young and in an unfamiliar place. The pilot ejects from the plane and had landed in the bushes up high. The movement of the pilot caused by the wind jerking him back and forth frightens the two boys.
"When the breeze blew, the lines would strain taunt and some accident of this pull lifted the head and chest upright so the figure seemed to peer across the brow of the mountain."(96)
i think a dead parachutist