Child behavioural and mental health problems have become a public health crisis. The consequences of poor mental health in childhood have large economic costs and consequences for the individual, their families, and for society. Early intervention through parenting programmes can reduce the onset of poor mental health in childhood, hence evaluating the effectiveness of parenting programmes is critical. The ‘Incredible Years Toddler’ parent programme is an education and training intervention designed to enhance the social and emotional wellbeing of children aged 1–3 years. Whilst previous studies show Incredible Years Toddler to provide promising effects on child outcomes in the short term, the research samples have lacked ethnic diversity and representation from socioeconomically deprived families. This quasi-experimental study is registered on ISRCTN (ISRCTN49991769). We will investigate the effectiveness of Incredible Years Toddler being delivered in three neighbourhoods in inner city Bradford, England. These neighbourhoods contain a socially and ethnically diverse population with 84% living in the poorest decile for England and Wales. Parents with a child aged 1–3 years old who are enrolled in Born in Bradford’s Better Start interventional family cohort study are eligible for this study. Intervention participants will be matched to a demographically comparable control group using propensity score matching. This study will use retrospective and prospective data from participants who attended Incredible Years groups between September 2018 and April 2024. The required minimum sample is n = 1336 (ratio 1:3) to detect a small effect (odds = 1.5, d = .20) on the Early Years Foundation Stage profile total score at age 5; a measure of early child development that is routinely collected by teachers. We will also establish whether these effects are moderated by child age at entry to intervention, programme delivery mode, socioeconomic status, and ethnicity. We will also estimate the cost of the intervention and conduct a cost-consequence analysis.