Children are particularly vulnerable to environmental tobacco smoke (ETS). There is no routine support to reduce ETS in the home. We systematically reviewed trials to reduce ETS in children in order to identify intervention characteristics and behaviour change techniques (BCTs) to inform future interventions. We searched Medline, EMBASE, CINAHL, PsycINFO, ERIC, Cochrane Central Register of Controlled Trials, and Cochrane Tobacco Addiction Group Specialised Register from January 2017 to June 2020 to update an existing systematic review. We included controlled trials to reduce parent/caregiver smoking or ETS in children <12 years that demonstrated a statistically significant benefit, in comparison to less intensive interventions or usual care. We extracted trial characteristics; and BCTs using Behaviour Change Technique Taxonomy v1. We defined “promising” BCTs as those present in at least 25% of effective interventions. Data synthesis was narrative. We included 16 trials, of which eight were at low risk of bias. All trials used counselling in combination with self-help or other supporting materials. We identified 13 “promising” BCTs centred on education, setting goals and planning, or support to reach goals. Interventions to reduce ETS in children should incorporate effective BCTs and consider counselling and self-help as mechanisms of delivery.