Changes in physical activity levels and increases in screen time across different cohorts may affect young children's motor skill development (MSD). This study examined cohort differences in MSD in China. Four-year-olds living in Shanghai and in Guizhou province were assessed, in individual sessions, in 2013 (n = 230) and in 2017 (n = 446). Controlling for age, parental education, gender, height, weight, and overall development, children's MSD scores were significantly lower in 2017 than in 2013 in both Shanghai and Guizhou. Parental education moderated these differences. Declines in MSD scores may be due to increased screen time, higher levels of air pollution, and decreased access to safe, outdoor spaces. The overall decline in motor skills across both regions suggests that appropriate and timely interventions are needed to support young Chinese children's MSD.