Which littlun gets sand in his eye? Which character feels bad about it?

Percival is the littlun who gets sand in his eye. Percival, along with two other littluns, Johnny and Henry, are spending time on the beach, playing with their sandcastle towns in quiet concentration, until two of the big kids come along. Roger and Maurice wander down to the beach for a swim, after they are relieved of their duty at the fire. Roger leads the way "straight through the castles, kicking them over, burying the flowers, scattering the chosen stones," but as the littluns are preoccupied with another section of their town, they do not protest the destruction. Maurice follows Roger and gleefully kicks away at the castles as well, which results in an eyeful of sand for Percival.
When Percival begins crying, Maurice is acutely reminded of his previous civilized life, referred to as "his other life" in the text, and immediately begins to feel guilty. The reader is told that Maurice was previously in trouble for getting sand in a younger child's eye, and the repetition of that action on the island reminds him of that previous reprimand. Even though they are isolated on the island, and there are no adult figures to punish him, "Maurice still felt the unease of wrongdoing." He immediately comes up with a half-hearted reason to flee the situation and takes off towards the water for a swim. It's no apology, but in this early stage of the story, the rules enforced on him back in his "other life" still hold a partial sway over his actions, enough that he feels some guilt for hurting Percival.