The answer is: This research would be eligible for exemption because the researcher is not interacting with the children and the playground is a public setting.
Approval from IRB is only needed if the research is being done on a controlled environment or the researchers make any contact/intervention to the subjects. IRB exist to ensure that all the changes made by the researchers to the subjects would be completely safe.
Public places cannot be considered a controlled environment by the researchers and simple observation at natural state would not make any change to the subject. Because of this, This research would be eligible for exemption