(1) Background: Participation in physical activity is crucial for the healthy growth and development of young children. More robust measurement of environmental influences on children’s physical activity in early childhood education and care (ECEC) settings may help resolve inconsistencies in the literature. This study evaluated the reliability of an environmental audit and educator practice survey for assessing ECEC physical, policy, and social environments related to young children’s physical activity. (2) Methods: A convenience sample of 20 ECEC centres participated in this PLAYCE (Play Spaces and Environments for Children’s Physical Activity) sub-study. Trained auditors conducted audits and educators completed surveys, two weeks apart. Test-retest reliability of the survey (n = 32), inter-rater (n = 20 pairs) and intra-rater reliability (n = 38) of the audit was assessed using intra-class correlation coefficients (ICCs), Kappa statistics and percent agreement. (3) Results: Intra-rater and inter-rater reliability ICCs for outdoor equipment, spaces and features were good to excellent (ICC = 0.70–0.94), while ratings for indoor equipment, media and spaces varied from fair to excellent (ICC = 0.46–0.78). The majority of items were rated by Kappa as moderate or above for intra-rater, inter-rater and survey test-retest reliability. (4) Conclusions: The PLAYCE Study instruments provide reliable measures of ECEC physical activity environments which can help to better understand influences on young children’s physical activity.