This paper highlights the socio-economic disadvantage experienced by disabled young children in England. Establishing cause or effect is always complex, but by using multiple measures from a longitudinal birth cohort study, we are able to shed new light on the lives of disabled children. We use the Millennium Cohort Study (MCS) to first enhance understanding of what constitutes disability, showing the prevalence of disability among children using three different definitions: developmental delay (DD), long-standing limiting health conditions or illnesses (LSLI) and special education needs (SEN). We found surprisingly little overlap between these three measures of disability. This highlights the heterogeneity among disabled children and the implications of using different forms of grouping or classifications to mark boundaries between disabled and non-disabled. More disabled children, however defined, were born into socio-economically disadvantaged circumstances that continued through their early years. Looking longitudinally, by age seven the disparities between disabled and non-disabled children had widened. The large sample size available also allowed us to highlight differences in the experience of socio-economic disadvantage among children identified with different special education needs. We found that socio-economic disadvantage was strongly associated with certain SEN conditions, such as behaviour, learning or speech and language difficulties, but was not associated with dyslexia.