chapter four.
It was in chapter 4 when Roger and Maurice kick down the littluns' sandcastles, afterward Maurice feels bad about it.
Roger knocks over the sandcastles.
Roger tramples through the littluns' sandcastles quite deliberately because he enjoys causing pain and suffering to other boys. Roger is a sadist who gets a kick out of being a bully. Maurice follows him because he is also a bully, but unlike Roger he is also a coward. He is happy to follow Roger's lead but lacks the nerve to do things by himself. Even while kicking down the sandcastles Maurice is mentally preparing an excuse in his mind.
Roger
Roger throws stones at the littluns, destroying their sand castles and causing them to feel intimidated and scared.
Maurice and Roger throw rocks at the castles the boys are building, destroying their hard work and causing frustration among the group. This destructive behavior underscores the growing tensions and breakdown of order among the boys on the island.
Maurice
Ralph, Jack, Roger, Piggy, Simon, Samn'Eric, Percival Wemys Madison, The littluns', Naval officer. Total characters: 9 + littluns'
If you really mean 'Who are the littluns' then the answer is simply that the littluns are the younger boys aged around six years old or so, some a little older. There is no specific age at which boys cease to be littluns and are counted as biguns. Apparently Simon, Roger and Maurice are in the transition area but they are all counted as biguns and their ages are never stated.
Roger is described as being a questionable and seedy character at the beginning of the book. He is also generally mean, he destroys the "littluns" and throws rocks at Henry (missing on purpose).
They were relieved from duty at the fire and had come down for a swim.
Roger and Maurice destroyed the sand castle out of a desire to exert power and control over the littluns, showing their inclination towards mischief and bullying.