Short story about child in a simulation and a pumpkin

Pretty sure this was a short story. There is a child who likes riding his bicycle around town. One day (for halloween) his mother asks him to go buy a Pumpkin, and he rides out of town and buys a Pumpkin. He wasn’t normally allowed to do this but he got distracted.

When he gets back home, the pumpkin is opened and is empty. The "parents" tell the child the truth, he is living in a simulation (and the parents are one too) and he is the last child on earth.