Roger and Maurice are intimidating the littluns who are enjoying their freedom at the beach. The older boys soon took it to far and destroyed the littluns sandcastles that they put considerable effort into.
Roger sneakingly follows Henry to where he is playing with the fish.
Then to add insult to injury, he begins to throw stones in his direction, purposely missing the boy. In the environment Roger was raised it was seen as a taboo (restricted by social customs) to throw rocks at someone. So even when in a place with no adult authority or rules, Roger wasn’t completely blind or forgetful of the values he was taught in his “old life”, which was a title that the author gave the boy’s home.