Little is known about how parents and children perform physical activity together. Parents are described as gate-keepers of children’s physical activity. We explored gate-keeping in cross-generational physical activity from child, parent, and family perspectives. A qualitative methodology was employed. Children and parents took part in semi-structured focus groups, family unit interviews, and individual interviews. Data was analysed thematically. The starting of cross-generational physical activity involved co-construction. Participants were found to instigate and respond to invitations from their physical activity partners. Child participants were found to employ several strategies to influence their parents and open-the-gate-on physical activity. In contrast, stopping cross-generational physical activity only required one participant to stop the physical activity for the physical activity to cease, and child participants volunteered no strategies to influence their parents during the stopping of cross-generational physical activity. Children of this age group were not passive recipients of parental gate-keeping in cross-generational physical activity. Cross-generational recreation, sport, and exercise was co-constructed by interplay between the child and parent. Physical activity health promotion policy and practice should recognise and promote the roles that both children and parents plays in the instigation of, and engagement in cross-generational physical activity.