Many children with cerebral palsy (CP) have muscle contractures and bony deformities. It has been hypothesised that these musculoskeletal abnormalities could increase or decrease muscle moment arms and cause movement dysfunction. In this study, we first investigated the relationship between skeletal growth (tibia length) and three-dimensional measurements of Achilles tendon and tibialis anterior moment arms from magnetic resonance images of 200 typically developing children aged 5 to 15 years. Moment arms increased linearly with tibia length. To determine whether cerebral palsy affects moment arms, we also measured Achilles tendon and tibialis anterior moment arms from a predominantly ambulant cohort of 79 children with CP (94% Gross Motor Function Classification System Level I or II). After adjusting for tibia length, age and sex, the Achilles tendon moment arms of children with cerebral palsy were, on average, slightly greater than those of typically developing children (mean difference 1.4 mm, 95% confidence interval (CI) 0.4 to 2.4;​ p < 0.01) and tibialis anterior moment arms were slightly smaller (mean difference −0.9 mm, 95% CI −1.6 to −0.2, p < 0.05). We conclude that moment arms scale linearly with tibia length during childhood development from 5 to 15 years. Ambulant children with cerebral palsy have moment arms that differ slightly, on average, from the moment arms of typically developing children, but those differences are too small to cause significant ankle joint dysfunction.