Little is known about whether and how parents can foster their children's spontaneous focus on number, an unprompted measure of attention to small numbers of objects that predicts later math achievement. In the current study, we asked 54 preschool-aged children and their parents to play together in a children's museum exhibit using either a numerical prompt or a nonnumerical prompt (control condition). Before and after playing with their parent, children completed assessments to measure individual differences in their tendency to spontaneously focus on number. After playing with their parent, children whose parents received the numerical prompt showed greater spontaneous focus on number compared to children whose parents received the control prompt. These findings suggest that when parents interact in an informal play setting with their children in ways that involve numerical content, it sharpens children's later spontaneous attention to numerical information.