Jerry's mother feels very conflicted in terms of how much freedom she should allow her eleven-year-old son. She would prefer to keep him with her on the safe beach, but she also realizes that she needs to allow him greater independence than she used to because he will require this freedom in order to learn how to operate in the world. She thinks,
Of course he's old enough to be safe without me. Have I been keeping him too close? He mustn't feel he ought to be with me. I must be careful [....]. She was determined to be neither possessive nor lacking in devotion. She went worrying off to her beach.
On the one hand, she wants to protect Jerry from the world (and from himself and the possibility that he will make poor decisions). Such a concern seems justified given Jerry's decision to swim through the rock at all costs, a move which nearly kills him. On the other hand, Jerry's mother knows that a child his age needs freedom from her, and she "conscientiously worr[ies] over what amusements he might secretly be longing for which she had been too busy or too careless to imagine."
Her internal conflict seems pretty typical of any parent of a child this age. Parents want so much to protect their children, but simultaneously realize that their children must experience independence in order to learn how to live in the world.