This study aimed to measure tripartite group area using motion capture systems and investigated whether group area could be used as a measure of pre-school children’s social interactions. In Experiment 1, two typically developing girls and an adult staff member engaged in free play. In Experiment 2, two typically developing boys and two adult staff members played balloon volleyball. Both experiments had three types of measures: subjective evaluation of whether participants played together, social behaviours (e.g. eye contact for Experiment 1 and balloon tosses for Experiment 2) and group area. Results showed that group area was significantly and negatively related to subjective evaluation in Experiment 2, whereas we observed no relationship between subjective evaluation and group area in Experiment 1. Overall, however, only a low correlation was observed between subjective evaluation and group area in Experiment 2. Furthermore, there were strong sequential associations between subjective evaluation and social behaviour, rather than between subjective evaluation and group area. Although group area as an index of social interactions is less accurate than behavioural data directly observed by humans, it may be worth using as a low-cost preliminary measure, since it can be automatically calculated using motion capture systems.