This study explored parents' physical activity (PA) knowledge and perceptions of children's out-of-school PA to formatively contribute to a family-based intervention design. Eleven telephone interviews were conducted with parents of children aged 10-11 years. Child and parent data were triangulated and family case studies were written. Most parents were unaware of the UK child PA guidelines and whether their child achieved the guidelines daily. PA for many parents was attributed to healthy weight status, and the neighbourhood environment was perceived as unconducive to children's outdoor play which consequently increased the attractiveness of adult supervised organized activities. Consulting with parents in a formative sense prior to familial PA intervention facilitates intervention content to be aligned with family-specific perceptions and needs, and offers opportunities to communicate the relevance of programs to parents. This may aid subsequent intervention recruitment and engagement.