Sedentary behaviour is emerging as an independent risk factor for paediatric obesity. Some evidence suggests that limiting sedentary behaviour alone could be effective in reducing body mass index (BMI) in children. However, whether adding physical activity and diet-focused components to sedentary behaviour reduction interventions could lead to an additive effect is unclear. This meta-analysis aims to assess the overall effect size of sedentary behaviour interventions on BMI reduction and to compare whether interventions that have multiple components (sedentary behaviour, physical activity and diet) have a higher mean effect size than interventions with single (sedentary behaviour) component. Included studies (n = 25) were randomized controlled trials of children (<18 years) with intervention components aimed to reduce sedentary behaviour and measured BMI at pre- and post-intervention. Effect size was calculated as the mean difference in BMI change between children in an intervention group and a control group. Results indicated that sedentary behaviour interventions had a significant effect on BMI reduction. The pooled effect sizes of multi-component interventions (g = −0.060∼−0.089) did not differ from the single-component interventions (g = −0.154), and neither of them had a significant effect size on its own. Future paediatric obesity interventions may consider focusing on developing strategies to decrease multiple screen-related sedentary behaviours.