The story you’re recalling is “Just Deserts,” a short story by M. T. Anderson. It appears in The Chronicles of Harris Burdick (2011), an anthology edited by Chris Van Allsburg where various authors wrote tales to match Van Allsburg’s mysterious illustrations. In “Just Deserts,” a lone child is raised in a simulated town after an apocalyptic event (implied nuclear war). He attends “school” and interacts with other children via screens, only to discover – in a scene involving a hollow pumpkin – that his parents and classmates are all artificial constructs created to keep him company. This twist reveals he is the last real child on Earth. The story was first published in The Chronicles of Harris Burdick (Houghton Mifflin, 2011).