The goal of this thesis is to understand why children with cerebral palsy (CP) expend more energy and fatigue faster when walking than their typically developing peers. I used a large observational database from children with CP in combination with causal and Bayesian statistics to accomplish this goal. For my first project, I estimated the causal effects of various biomechanical and neuromuscular factors on walking metabolic power. Metabolic power was most influenced by impaired kinematics and motor control. In my second project, I quantified the time rate of oxygen kinetics in children with CP. I found they had slower oxygen kinetics than what has been reported for children with typical development. This indicates they might also have greater metabolite flux from greater anaerobic energy production and could explain faster rates of fatigue. Together, these projects suggest that biomechanical and physiological differences underlie greater energy expenditure and fatigue in children with CP.