Background: An area of growth in physical activity research has involved investigating effects of physical activity on children's executive functions. Many of these efforts seek to increase the energy expenditure of young children as a healthy and low-cost way to affect physical, health, and cognitive outcomes. Methods: We review theory and research from neuroscience and evolutionary biology, which suggest that interventions seeking to increase the energy expenditure of young children must also consider the energetic trade-offs that occur to accommodate changing metabolic costs of brain development. Results: According to Life History Theory, and supported by recent evidence, the high relative energy-cost of early brain development requires that other energy-demanding functions of development (ie, physical growth, activity) be curtailed. This is important for interventions seeking to dramatically increase the energy expenditure of young children who have little excess energy available, with potentially negative cognitive consequences. Less energy-demanding physical activities, in contrast, may yield psychosocial and cognitive benefits while not overburdening an underweight child's already scarce energy supply. Conclusions: While further research is required to establish the extent to which increases in energy-demanding physical activities may compromise or displace energy available for brain development, we argue that action cannot await these findings.