Who is in control of the relationship, the boy or the mother?
I think they both have their moments, but if I were going to have to pick one, I would say that the boy is the one who is in control of this relationship. I think that he knows a bit better how to manipulate his mom.
We see, for example, that she will get him goggles just because he wants them. We also see, more importantly, that he is willing to go off on his own without asking permission. I think we can also see it at the very end of the story where the mom thinks she understands what is going on and is ready to argue. But the boy is really in control and he has already decided that he's done swimming.
I agree with the previous editor. The mother arranged the trip and tells the child what to do. He listens to his mother but soon gets bored and curious when he spots the other boys. He is yearning for a friend and so he tries to play with the other boys. He wants camaraderie.
The boy takes control when he does not tell his mother that he is going to go through the tunnel. Had she known she would have probably not let him go, but he does not inform her. Instead, he takes a life and death risk on his own.
Getting through the tunnel was difficult but when he returns he has a secret that is his own. He has proved himself worthy of hanging with the others and recognized hs own strength.