Little is known about what drives engagement in physical activity involving children and parents together. To date, when this phenomenon has been studied, the focus has been upon parent support for child physical activity, ignoring the child perspective. This article explores child and parent drivers of cross-generational physical activity. Methods: A qualitative, hermeneutic methodology was employed. Primary school children and parents took part in semi-structured focus groups, family unit interviews, and individual interviews. Data was transcribed and analysed thematically. Results: Engagement in cross-generational physical activity was driven by much more than physical health benefits. Emotional and relational drivers of cross-generational physical activity were identified, highlighting the mostly positive impacts it had upon both family and child–parent relationships by providing connecting and bonding experiences. Children identified it as a unique physical activity partnership, which provided a safe context for practising their sporting skills. Parents reported that cross-generational physical activity facilitated parenting by providing opportunities for them to teach and nurture important life skills, while also providing their child with support for physical activity. Holidays were identified as a time when the focus of physical activity for families was more often cross-generational. Conclusion: Cross-generational physical activity is a complex, bi-directional physical activity partnership that takes place within a child–parent relationship, within a family. Its drivers are manifold, extending well beyond the dose of physical activity it provides, to strengthening relationships and skill development. Future research should include the child’s voice to provide a more holistic view of this phenomenon.