how did the kids react when they see no grownups around?

Ralph is intrigued by the idea:

“Aren’t there any grownups at all?” “I don’t think so.” The fair boy said this solemnly; but then the delight of a realized ambition overcame him. In the middle of the scar he stood on his head and grinned at the reversed fat boy.

All of the boys are filled with the freedom no adults affords them. Some of the littl'uns are frightened, some cry, but on the whole the entire siituation is initially treated like an adventure.