This article examines the effects of socio-economic conditions on the standardized heights and body mass index (BMI) of children in Interwar Britain, using the Boyd Orr cohort, a survey of predominantly poor families taken in 1937-9. We examine the trade-off between child quality (in the form of health outcomes) and the number of children in the family. We find that birth order and family size have negative effects on the heights of children, but not on their BMI. Household income per capita positively influences height but, even after accounting for this, the number of children in the family has a negative effect on height. This latter effect is closely associated with overcrowding and with the degree of cleanliness or hygiene in the household, which conditions exposure to factors predisposing to disease. We also analyse follow-up data, which indicates that the effects of family size on height persisted into adulthood.