OBJECTIVES: This study examined the impact of hypothetical interventions on (i) early life factors (prenatal and early postnatal) on rapid weight gain during infancy, and (ii) early life factors and child energy balance-related behaviours (EBRB) on children's body mass index (BMI)-for-age at 5 and 8 years. METHODS: Data from the Norwegian Mother, Father, and Child Cohort Study included participants aged 2 (n = 48 109), 5 (n = 18 810) and 8 (n = 10 830) years. Early life intervention variables were maternal smoking during pregnancy, maternal weight before and during pregnancy, exclusive/partial breastfeeding for 6 months, and introduction of complementary food at 6 months. Child EBRB intervention variables for the 5-year outcome included screen time, fruit and vegetable intake, and sugar-sweetened soft drink intake assessed at 3 years. For the 8-year outcome, interventions included screen time, presence of a television in the child's bedroom, sleep hours and breakfast intake assessed at 5 years. The parametric g-formula was used for analysis. RESULTS AND CONCLUSIONS: The average population-level difference in rapid weight gain during infancy at 2 years under the intervention targeting the prenatal, early postnatal factors and these factors combined with 95% confidence intervals were -0.012 (-0.017, -0.007), -0.009 (-0.012, -0.005) and -0.020 (-0.025, -0.015), respectively. The average population-level differences in children's BMI-for-age z-score for interventions targeting early life factors, child EBRB and these factors combined were -0.225 (-0.244, -0.207), 0.02 (-0.021, 0.024) and -0.223 (-0.249, -0.196), respectively among 5-year-olds and -0.265 (-0.295, -0.236), -0.020 (-0.029, -0.011) and -0.285 (-0.315, -0.256), respectively among 8-year-olds. Our results suggested joint interventions on early life factors may reduce childhood BMI-for-age z-scores.