We estimate flexible child health production functions to investigate whether more hygienic environments, characterized by better water, sanitation, and hygiene (WASH) practices, make nutrition intake more productive for the physical growth of children aged 6-24 months. Using ten rounds of exceptionally rich longitudinal Filipino cohort data, we estimate value-added production functions with a control function approach. We show that WASH and nutrition are complements in the formation of child height and weight. Nutritional and WASH conditions faced by sample children are similar to those currently encountered by poor children in low-income settings with comparable stunting rates.