It is well known that physical activity is important for children’s current and future mental and physical health. Despite this, there appears to be a secular decline in children’s physical activity (Cameron et al. [2]; Dalene et al. [3]). Furthermore, (frustratingly) interventions aiming to increase children’s physical activity have limited success (Metcalf et al. [10]), demonstrating a need for more information on the amenability of activity levels to change.