Sleep is vital for well-being. During the COVID-19 pandemic, significant disturbances in family life impacted both stress and sleep, particularly in parents of young children. In the context of parent-child relations, both maternal sleep and child sleep are likely to have inordinate impacts on maternal physiological functioning. This study examines the longitudinal relationship between maternal and child sleep and maternal chronic stress, indexed by hair cortisol, during the early stages of the pandemic. Participants included 113 mothers (M[subscript age] = 37.20 years, SD = 5.23) and their children (females = 49%; M[subscript age] = 72.88 months, SD = 10.94). Mothers completed surveys on sleep and provided hair samples for the analysis of cortisol. Regression analyses demonstrated that after controlling for initial levels of cortisol, child sleep disturbance, but not maternal sleep problems, at Time 1 predicted maternal hair cortisol at Time 2 (about 6 months later; b = 0.283, SE = 0.107, p = 0.008). Child's sleep and its implications for maternal physiological health are discussed.