Links between heat exposures, maternal mental health and young child development are complex. Relationships between heat and maternal mental health and child development could be worsened by differences in home cooling access (air conditioning) given economic constraints. We examined these associations among young children in two US cities, using 2008–2020 survey data from Boston and Little Rock Children’s HealthWatch cohorts. Children’s HealthWatch is a study of young children from low-income families in hospital settings in 4 US cities. We assessed temperature and energy insecurity’s (difficulty affording utilities) relationship with child health, child development risk, and maternal depressive symptoms. Temperatures on the survey day and two weeks prior were associated with increased maternal depressive symptoms in Massachusetts. In Arkansas there were no relationships between temperature and maternal depressive symptoms, however, energy insecurity changed the relationship between them; in the two weeks prior to the survey, for every 1 degree C increase in temperature, energy-insecure households were 17% more likely to have maternal depressive symptoms. Temperature was not related to child health status in either city but had a potential relationship with developmental risk. Results are useful for extreme weather, housing and material hardship policy solutions at city, state, and federal levels.