•Head Start families were provided with mathematical card games to play at home.•Playing a shape and color matching card game improved children’s shape knowledge.•Duration and frequency of game playing related to children’s learning.•Parents used a range of guidance and assistance techniques during the games. Discrepancies in early mathematical knowledge between children from different socioeconomic backgrounds have been found before the start of kindergarten. The early home environment is one context that can address these discrepancies. This study examined whether an informal mathematical activity that has been successful at promoting children’s numerical knowledge could be translated into a home activity for families from lower-income backgrounds. Families from Head Start programs (n = 39) were randomly assigned to play either a numerical magnitude comparison game or a shape and color matching game. Results showed that playing the numerical magnitude comparison game did not improve children’s numerical knowledge, although playing the shape and color matching game did improve children’s shape knowledge. However, parental reports of the frequency of game playing at home related to children’s learning from both games. Analyses of audio recordings of the families playing the games at home revealed there was wide variability in how parents assisted the children during the card game play. Results are discussed in terms of the benefits and challenges of mathematical interventions targeting the home context.