The boy and his mother in Lessing's "Through the Tunnel," have a tense relationship. That doesn't, however, mean that it is in any way abnormal.
The mother is naturally overprotective. Her tendency is to shelter her son and try to protect him. The boy, like any young boy or adolescent, wants to be independent.
Though she is unsure, the mother allows the boy to go off on his own to the part of the beach that is not commonly frequented by swimmers; she lets him go off swimming in an unmarked area, probably.
The boy is obsessed with fulfilling the task of swimming through the tunnel, and fights back hard when anything gets in the way--including his mother, when he childishly demands she buy him goggles immediately.
Yet, look how relaxed the relationship becomes at the close of the story. The mother is expecting a battle when she tells him he cannot go back to the bay. But he goes along easily. He's fulfilled his quest, and maybe matured a bit.