Background Parental smoking is associated with an increased risk of smoking among youth. Epidemiological research has shown that parental smoking cessation can attenuate this risk. This study examined whether telephone counselling for parents and subsequent parental smoking cessation affect smoking-related cognitions and smoking initiation among children of smoking parents. Methods Data of a two-arm randomized controlled trial were used in which 512 smoking parents were recruited into cessation support through their children's primary schools. After the baseline assessment, smoking parents were randomly assigned to tailored telephone counselling or a standard self-help brochure. Parental cessation was measured as 6-month prolonged abstinence at the 12-month follow-up. Children's smoking-related cognitions and smoking initiation were examined at 3-month, 12-month, and 30-month follow-up. Results No statistical evidence was found that children of parents who received telephone counselling tailored to smoking parents or children of parents who achieved prolonged abstinence differ in smoking-related cognitions (i.e., smoking outcome expectancies, perceived safety of smoking, self-efficacy to refrain from smoking, susceptibility to smoking) or smoking initiation rate on any follow-up assessment. Conclusions This study is the first to examine the effects of an evidence-based smoking cessation treatment for parents and treatment-induced parental smoking cessation on cognitive and behavioural outcomes among children. Although descriptive statistics showed lower smoking initiation rates among children of parents who achieved prolonged abstinence, there was no statistical evidence that telephone counselling tailored to parents or treatment-induced parental smoking cessation affects precursors of smoking or smoking initiation among youth.