There is an increasing understanding of the role of the built environment on children's neighbourhood mobility and play and the importance of this for children's development. This has led to concerns over children's declining neighbourhood mobility and calls to see an increase in children's use of public space. This paper draws on findings from a research study working with 9 and 10 year olds living in inner London, England. The children participated in go-along interviews and a range of other qualitative methods, which explored how they used their neighbourhoods for getting around and play. Findings from the study demonstrate the importance of threshold spaces for children in supporting both their neighbourhood play and their wider neighbourhood travel and mobility. Threshold spaces are defined as a semi-public space that straddle the gap between the private space of the home and the wider public realm. Children's use of threshold spaces was influenced by a reduced movement function in these spaces, restricting vehicles and people passing through, and the presence of signals that it was ok to play, with girls being more sensitive to these features than boys. Threshold spaces were important as a start point for children's wider explorations of their neighbourhoods.