This paper presents a dynamic model of child labour supply in a farming household. The model clarifies the roles of land, income and household size, allowing labour and credit market imperfections. If labour markets are imperfect, child labour is increasing in farm size and decreasing in household size. The effect of income is shown to depend upon whether the effective choice is between work and school or whether leisure is involved. Credit market constraints tend to dilute the positive impact of farm size and reinforce the negative effect of income. The model is estimated for rural Ghana and Pakistan. A striking finding of the paper is that the effect of farm size at given levels of household income is significantly positive for girls in both countries, but not for boys. This is consistent with the finding, in other contexts, that females exhibit larger substitution effects in labour supply. Increases in household income have a negative impact on work for boys in Pakistan and for girls in Ghana but there is no income effect for the other two groups of children. We find interesting effects of household size and composition, female headship, and mothers' post-secondary education.