The aim of this study was to systematically review the protocols to assess the built environment in the context of early childhood and physical activity. Three databases were systematically searched for studies that examined associations between the built environment and preschoolers’ physical activity levels. A total of 27 studies were included in the analysis. Results show a clear distinction between approaches administered in the neighborhoods and childcare domains. Although Geographic Information Systems are employed in most studies in the neighborhood domain and have been understood as an objective approach, findings have indicated a lack of specificity regarding early childhood environmental variables, which can compromise protocols content validity. Further, although self-perceived approaches were found in only three studies, analysis of instrument content validity and cross-cultural adaptation also showed limited validation evidence. In the childcare context, the micro-scale built environment is a strong trend in customization of measurement protocols in early childhood. However, present findings have indicated the necessity for enhancements to validation procedures, particularly regarding internal consistency and content validity to develop needed customization. Therefore, future research should utilize more rigorous validation techniques to address the micro-scale built environment within the context of early childhood physical activity levels.