Background Childhood shapes lifelong wellbeing, making it crucial to understand how environmental factors impact development. This study examines the impact of neighbourhood greenspace quantity on the trajectories of emotional, behavioural and cognitive outcomes across childhood (at ages 3, 5, 7 and 11 years) with data from the UK's Millennium Cohort Study. Methods Using multilevel growth curve models, we assessed the role of neighbourhood greenspace in small standard areas on trajectories of conduct problems, hyperactivity/inattention, peer problems, emotional symptoms and cognitive ability. Results There was no direct association between greenspace and these child outcomes at the intercept (~ aged 7 years). However, greenspace was related to the slope of both conduct problems and cognitive ability, suggesting possible benefits in the early years, mainly before the start of formal education. Conclusion The study highlights the potential effect of greenspace quantity on child development, but in the context of age. Longitudinal research tracking outcomes beyond childhood can shed more light on age‐related effects of greenspace across areas of development.