Unhealthy weight, especially childhood obesity, is emerging as a growing epidemic and a challenge in developed and developing countries. Partnership with parents to promote healthy lifestyle changes may have a lifelong impact on weight-related outcomes in children. This study aims to determine the efficacy of an Interactive Malaysian Childhood Healthy Lifestyle (i-MaCHeL) intervention programme to change weight-related behaviour in preschool child-parent dyads. The i-MaCHeL programme is a single-blind, theory-driven intervention, two-group cluster randomised controlled trial that evaluates the efficacy of a 3-month health promotion intervention in preschool child-parent dyads. In recognition of the value of multiple theoretical approaches, the strong theoretical basis consists of Social Cognitive Theory, Health Belief Model, and Trans-Theoretical Model principles underpinning the development of the intervention programme. In total, 460 child-parent dyads from 12 preschools in Terengganu, Malaysia, will be recruited. The children in the intervention group will expose to the i-MaCHeL classroom activities, while the parents will access the i-MaCHeL Web-based educational programme and numerous parent-child home-based online activities. The children in the control group will continue with any existing health-related activities, while the parents will receive the link to the general health newsletters. BMI z-score, dietary intake, physical activity, screen time duration, health-related quality of life, parental self-efficacy, parental role modelling, and parental policies will be assessed at baseline, 3 months' post-baseline, and at 6 months' follow-up (9 months' post-baseline). General linear model repeated measure analysis will be used to determine differences between groups at the 3- and 9-month surveys with adjustment for potential covariates. Statistical analyses will follow intention-to-treat principles. We hypothesise that the combination of the classroom and interactive Web-based activities will have a strong potential to be effective strategies to sustain child-parent engagement and participation in the weight-related behaviour change programme.