Energy education programmes for children are hypothesized to have great potential to save energy. Such interventions are often assumed to impact child and family behaviours. Here, using a cluster-randomized controlled trial with 30 Girl Scout troops in Northern California, we assess the efficacy of two social cognitive theory-based interventions focused on residential and food-and-transportation energy-related behaviours of Girl Scouts and their families. We show that Girl Scouts and parents in troops randomly assigned to the residential energy intervention significantly increased their self-reported residential energy-saving behaviours immediately following the intervention and after more than seven months of follow-up, compared with controls. Girl Scouts in troops randomly assigned to the food-and-transportation energy intervention significantly increased their self-reported food-and-transportation energy-saving behaviours immediately following the intervention, compared with controls, but not at follow-up. The results demonstrate that theory-based, child-focused energy interventions have the potential to increase energy-saving behaviours among both children and their parents. Behavioural interventions targeting children can influence the whole family, making them attractive for energy-saving schemes. Boudet et al. use 30 Girl Scout groups to test interventions on residential and food and transport energy behaviours and find they have potential for increasing energy saving.