A number of studies demonstrates a relationship between neighbourhood concentration of affluence and disadvantage and the health and development of its residents. We contribute to this literature by testing hypotheses about the relationship between neighbourhood-level concentrated affluence/disadvantage and child-level developmental outcomes in a study population of 37,798 Kindergarten children residing in 433 neighbourhoods throughout the province of British Columbia, Canada. We utilise a previously-validated measure of neighbourhood socioeconomic composition--the Index of Concentration at the Extremes (ICE)--which not only allows for more precise estimation of the competing influences of concentrated affluence and disadvantage, but also facilitates examination of the potential impact of neighbourhood-level income inequality. Our findings show that increases in neighbourhood affluence are associated with increases in children's scores on the Early Development Instrument (EDI), a holistic measure of Kindergarteners' readiness for school. Particularly noteworthy is that, for four of the five EDI scales (physical, social, emotional, and communication) and the total score, results indicate a significant curvilinear relationship--whereby the highest average child-level outcomes are not found in locations with the highest concentrations of affluence, but rather in locations with relatively equal proportions of affluent and disadvantaged families. This finding suggests, first, that concentrated affluence may have diminishing rates of return on contributing to enhanced child development, and, second, that children residing in mixed-income neighbourhoods may benefit both from the presence of affluent residents and from the presence of services and institutions aimed at assisting lower-income residents. Implications and future directions are discussed.