This paper follows an earlier report of young children's object play activities investigated in a cross-sectional sample of 289 typically developing children. Thirty-minute videotaped observations were taken of children at 8, 12, 18, 24, 30, 36, 42, 48, 54, and 60 months of age in their homes. Forty-nine percent were boys. Children were identified as White (70%); mixed racial/cultural backgrounds (14%); Asian (7%); Black (6%); and Latinx (3%). This study reports patterns of cognitive development based on mastery scores derived from frequency and variety of activities in the 27 categories identified in the earlier study. Mastery scores afforded evaluation of the robustness of certain play categories, and the retention and reduction of others, yielding a final set of 14 to describe and assess developments in play. Discussion centers on the value of differentiated categories along with scores that yield a standard for an assessment of children's play.