Children’s social interactions are crucial for healthy and adaptive social and emotional development. Free play situations, like the school break, represent an important opportunity for children as they learn how to negotiate, take their losses, be a part of a group, and not stand alone. Yet how to measure children’s social interactions in groups outside or at large playgrounds? Traditionally, questionnaires and observations are used in order to gain a systematic insight into children’s social behavior when they move around in groups; however, these methods can be both unreliable and intrusive. To increase the construct validity of these measures, we have applied a new method to follow children’s dynamics at the playground, using radio frequency identification devices (RFID). In this case, we describe how this method works, what it can measure, how it adds to current methods, and the limitations of each of these measures (questionnaires, observations, and RFID sensors). Finally, we give suggestions for the use of RFID sensors and for further development in this area.