The income contribution of child work is undoubtedly a key factor influencing child work and schooling decisions. Yet, few studies have attempted to directly measure this contribution. This is particularly the case for work performed on the household farm, as is the case for the vast majority of child workers, rather than for wages. In this study, we estimate a household income function with child labour included as an input. Results using a variety of functional forms and alternative child labour variables are compared. We conclude that children and adults are perfect labour substitutes and that the marginal productivity of children is roughly one-third to one-half that of male adults. The average contribution of each working child is estimated at 4 to 7% of household income, although there is substantial variation with contributions ranging up to 50%. These results underline the dependency of poor households on child work for survival.