Early years experiences shape a child's physical, cognitive and emotional development. Spending time in greenspaces offers benefits for children's development, but access and use can be limited in urban settings. There is increasing interest in the health and developmental benefits of Forest Schools for primary-aged children, but little is known about the benefits for pre-school children. This study aims to identify these and explore the processes and activities associated with a Forest School intervention for early years children that may influence outcomes. This paper reports on an ethnographic approach involving 65 hours of observations with two cohorts of 10 3-year-olds attending 11 weekly Forest School sessions in an urban setting. The children attending had little or no previous experience of natural spaces. 14 in-depth interviews were conducted with parents, and nursery and Forest School staff. The data were analysed using thematic analysis, and outcomes were identified using the Early Years Foundation Stage Statutory Framework. Despite logistical challenges, the intervention benefitted age-specific health and development outcomes, particularly personal, socio- and emotional development, verbal communication, and mathematics. Unexpected benefits were observed among nursery staff and parents attending Forest School. Forest Schools are a promising and feasible method to improve nature connectedness and development in children aged 3 years and support school readiness. The maintenance and protection of urban woodland spaces are paramount to facilitate this.