“Active 1 + FUN” is a family-based and self-determination theory-guided intervention program, designed to increase children's and parents’ physical activity in leisure-time. In this qualitative study, we aimed to provide a comprehensive understanding of the intervention by exploring perceived behavioral and psychological changes, and implementation of the program from the perspective of facilitators and participants. The interviews and focus groups explored facilitators’ and participants’ (children and parents) perceptions of the intervention in terms of the theoretical basis, perceived behavioral changes, basic need satisfaction (i.e., competence, autonomy, and relatedness), and other aspects. The findings suggested that facilitators, children and parents perceived the theory-driven intervention to be effective in increasing family's leisure-time co-physical activity and basic psychological need satisfaction.