Abstract: Children in modern societies are experiencing a lack of play in natural settings, which leads to not only play deprivation but also health issues such as the diminished use of the senses, attention difficulties, and higher rates of physical and emotional illness. Playground trees can play a vital role in promoting the health and developmental benefits for young children in cities. This study aimed to examine the impacts of trees and greenery of urban outdoor playgrounds on children’s play behaviors and outdoor play time in the United States. Two recent comparative case studies were documented. Study I compared two outdoor playgrounds in Washington, DC, with different levels of access to nature. Following the Play Observation Scale, 185 children’s play behaviors were observed and mapped through the Geographic Information System (GIS). Study II compared two outdoor playgrounds in a themed children’s park in the city of Tulsa, Oklahoma, in which 94 children’s play behaviors were tracked and observed. Findings indicated that children’s play behaviors varied by age and gender groups. The presence of nature and tree shade could significantly impact children’s social, cognitive, and free play behaviors. Playgrounds with sufficient tree shade predict the highest level of play behaviors in children from all age groups, especially social play and games with rules. Tree shade is also associated with longer outdoor play time, particularly for the 6 to 12 years age group. Considering gender and age as significant moderators, playground design that accommodates different groups of children could improve playground efficiencies. This research tested the effectiveness of a mixed-method research protocol of combining site observations with Lidar and GIS analytics. The findings highlighted the important roles of playgrounds’ design characteristics in shaping children’s play behaviors and called attention to the thermal comfort of children during play.