Promoting physical activities that involve both parents and their children would be very useful to the improved health and well-being of families, yet coactivity interventions have been particularly unsuccessful in past research. The purpose of this study was to elicit the salient parental beliefs about coactivity framed through theory of planned behavior in order to inform future intervention content. A representative sample of Canadian parents (N = 483) with children aged 6 to 14 years completed belief elicitation measures of theory of planned behavior, as well as coactivity and program preferences. Analyses included content theming by physical activity belief and preference through tallies of the percentages of parents endorsing each belief. Exploratory analyses of differences in endorsed themes were conducted by parent (mother, father), age of child (6-10 years, 11-14 years), and sex of the child. The results showed that behavioral beliefs about health, interpersonal and educational/learning opportunities and control beliefs about lack of time, various incompatible parent/child factors, parental health, and bad weather were dominant themes. Most of these themes did not vary in endorsement by parent and child characteristics. By contrast, preferences for various activities varied by parent and child characteristics, yet parents overwhelmingly desired the activities to be outdoors, close to home, after work, and originally delivered from community health professionals via Internet or face-to face means. Overall, the findings provide several considerations for specific targets to improve future physical activity intervention approaches among parents and their children.