Planned environments play a central role in children's lives, by creating opportunities and constraints for physical activity, mobility, and positive health outcomes. Urban planning can facilitate the creation of functional and inclusive environments for all young citizens. However, despite burgeoning global interest in children's well-being in cities, there remains little clarity or institutional support for children's needs or active role in urban planning processes. This chapter examines the history of degrees of separation between the idea of childhood (as a social construct) and developments within urban planning in the United Kingdom and beyond. By drawing links between the two, the chapter shows that planning can assume a leading role in facilitating children's freedom and independent mobility, addressing concerns around health, inequalities, human rights, and safety. This connects to increasing awareness around children's rights, engagement processes incorporating children's views, new methods of working with children, and the global acknowledgement of the multiple crises of childhood.