Children’s attention to numerosity is reported to uniquely predict their later mathematical skills (e.g., Hannula-Sormunen, 2015), but there is some debate concerning the extent to which this attention to number is spontaneous or contextually driven, and it is not known how attention to numerosity varies with respect to other mathematical features, such as shape. In the present study, we used a within-subjects observational design to examine attention to mathematical features, during brief semi-structured play sessions, among parent–child dyads. Our aims were to address whether children’s and parents’ attention to mathematical features varies with play materials and whether attention to mathematical features is correlated between children and their parents. Specifically, we coded verbal references to quantitative (number, quantity, and part-whole relations) and spatial (size, shape, location, and orientation) features that emerged during Duplos and kitchen set play among 45 two- to four-year olds and their parents. Analyses of variance revealed that children and parents attended to mathematical features more frequently during Duplos compared to kitchen set play, and that this difference was largely accounted for by greater attention to spatial features, but not quantitative features. Correlations between children’s and parents’ attention to mathematical features differed between Duplos and kitchen set play, suggesting that these play materials also influence mathematics-oriented play dynamics. The results extend previous experimental laboratory findings on attention to number to additional mathematical features, and provide evidence for the contextual influences on attention to mathematical features in naturalistic settings.