Abstract Background Objectively measured physical activity is low in British children, and declines as childhood progresses. Observational studies suggest that dog-walking might be a useful approach to physical activity promotion in children and adults, but there are no published public health interventions based on dog-walking with children. The Children, Parents, and Pets Exercising Together Study aims to develop and evaluate a theory driven, generalisable, family-based, dog walking intervention for 9-11 year olds. Methods/design The Children, Parents, and Pets Exercising Together Study is an exploratory, assessor-blinded, randomised controlled trial as defined in the UK MRC Framework on the development and evaluation of complex interventions in public health. The trial will follow CONSORT guidance. Approximately 40 dog-owning families will be allocated randomly in a ratio of 1.5:1 to receive a simple behavioural intervention lasting for 10 weeks or to a 'waiting list' control group. The primary outcome is change in objectively measured child physical activity using Actigraph accelerometry. Secondary outcomes in the child, included in part to shape a future more definitive randomised controlled trial, are: total time spent sedentary and patterning of sedentary behaviour (Actigraph accelerometry); body composition and bone health from dual energy x-ray absorptiometry; body weight, height and BMI; and finally, health-related quality of life using the PedsQL. Secondary outcomes in parents and dogs are: changes in body weight; changes in Actigraph accelerometry measured physical activity and sedentary behaviour. Process evaluation will consist of assessment of simultaneous child, parent, and dog accelerometry data and brief interviews with participating families. Discussion The Children, Parents, and Pets Exercising Together trial should be the first randomised controlled study to establish and evaluate an intervention aimed at dog-based physical activity promotion in families. It should advance our understanding of whether and how to use pet dogs to promote physical activity and/or to reduce sedentary behaviour in children and adults. The trial is intended to lead to a subsequent more definitive randomised controlled trial, and the work should inform future dog-based public health interventions such as secondary prevention interventions in children or adults. Trial registration number ISRCTN85939423