This qualitative case study was conducted to explore parent-child interaction in intergenerational play of numeracy-related Apps. A Chinese mother and her preschool-age son in a Midwestern university town were observed using several numeracy-related mobile game apps together. Our analysis of the observations and the interview with the mother revealed that parents might mediate preschooler’s App play, but their engagement might take a different shape than what Joint Media Engagement theory envisions. It provided insights about designing good App number-related games for parents and children to engage together.