Key PointsQuestionWhat role did children play in household viral transmission during the COVID-19 pandemic, when enveloped virus rates were low and relative proportions of COVID-19 were at a high?FindingsIn a cohort study of 166 170 households with adults and children using smart thermometers, among 38 787 inferred household transmissions over 3 years, 70.4% had a pediatric index case. Rates dropped during school breaks.MeaningThese results suggest that children were important viral vectors in households during the pandemic, particularly when school was in session.