Physical activity interventions among youth have resulted in modest outcomes; thus, there is a need to increase the theoretical fidelity of interventions and hone pilot work before embarking on large scale trials. The purpose of this study was to examine the effect of a planning intervention in comparison to a standard condition on intergenerational physical activity in families with young children. Inactive families (N = 85) were randomized to either a standard condition (received physical activity guidelines and a local municipal healthy active living guide) or the intervention (physical activity guidelines, local municipal healthy active living guide + planning material) after completing a baseline questionnaire package. Sixty-five families (standard condition n = 34; intervention condition n = 31) completed the 4 week follow-up questionnaire package. Complete cases and intention to treat analyses showed that the planning intervention resulted in higher self-reported family physical activity compared to the standard condition and this was due to an increase in unstructured family activities over the 4 weeks. The results are promising and suggest that theoretical fidelity targeting parent regulation of family activity may be a helpful approach to increasing weekly energy expenditure.