Prosocial behaviours such as helping, cooperating and sharing have numerous positive impacts on children’s social and emotional development. These behaviours can act as protective factors against the negative outcomes associated with antisocial behaviours, particularly amongst children at risk of adverse childhood experiences. While the use of behavioural supports, to promote prosocial behaviour, is well-established in formal school settings, our knowledge of their use in informal, community-based settings is limited. This thesis addresses this gap by exploring how group contingency interventions, interventions in which a common consequence is delivered to a group of children based on the behaviour of one or more individuals within that group, can be implemented in afterschool settings to promote prosocial behaviours of children. This research combines a systematic review exploring the use of group contingencies for targeting prosocial behaviours and antisocial behaviours and four empirical studies exploring the use of reinforcement-based group contingencies, the Caught Being Good Game, to increase prosocial behaviour in afterschool centres serving at-risk children. Aligned with participatory approaches and social validity, we strove to involve participants in meaningful ways through various novel methods. For example, in Chapter 4 the adult leader was involved in monitoring behaviour change using direct behaviour ratings and in Chapter 5, two studies are presented in which co-design methods were used to design the group contingency with adult leaders and children as key design partners.Findings suggest that the Caught Being Good Game can be effectively implemented in afterschool settings to target prosocial behaviours. Moreover, evidence suggests that participatory approaches can have positive implications for social validity from both adult leader and child perspectives. By centring the voices of participants and embedding co-design and participatory approaches within intervention design, this thesis presents a novel framework for implementing socially valid, trauma-informed behavioural strategies outside the classroom.