In that first paragraph, Jerry's mother "looked impatient, then smiled." She assumed he was right behind her, but he was dawdling and stopping to turn and look "toward the bay" instead. Her initial impatience seems to be a typical parental response when they believe their child is with them and turn around to find the child is lagging far behind, distracted by something or other.
When she sees where Jerry is looking, his mother asks, "'Why, darling, would you rather not come with me? Would you rather--,'" stopping short before she finishes the question. She doesn't honestly know what would amuse him, if those amusements would be ones she'd approve or feel were safe. "She frowned, conscientiously worrying over what amusements he might secretly be longing for which she had been too busy or too careless to imagine." As an eleven year-old boy, Jerry's interests are changing and he is beginning the process of becoming an adult, beginning to long for freedom and independence from his mother. Perhaps this moment sort of snuck up on her. She realized, of course, that this moment would come, but she worries now about how to treat him and what to say, "determined to be neither possessive nor lacking in devotion." Her worry is caused by the internal conflict she feels, wanting to grant Jerry the independence he needs but also wanting to keep him safe.