Objectives Social health inequalities remain a key policy challenge. The existing literature has not presented a synthetic view on the evolution of inequalities in physical development across childhood. We examine social disparities as children grow older using a range of different outcomes. Study design Population-based secondary data analysis. Methods We employ longitudinal data on British children aged 9 months to 12 years from the Millennium Cohort Study (n = 13,811–18,987) and focus on multiple child physical measures: weight, body mass index (BMI), overweight, fat mass and waist circumference. Results Higher family income is associated with lower BMI (for females), less body fat and a smaller likelihood of overweight (for both genders) on average throughout childhood. When income is multiplied by 3, the probability of overweight decreases by 2.8 (95% CI −0.041 to −0.016) percentage points for females and by 2.7 (95% CI −0.038 to −0.016) percentage points for males. Social inequalities in weight, BMI, overweight and body fat significantly widen as children grow older, for both genders. For instance, for females, when income is multiplied by 3, the probability of overweight decreases by 1.6 (95% CI −0.032 to −0.000) percentage points at ages 2–3 years, but by 8.6 (95% CI −0.112 to −0.060) percentage points at ages 10–12 years. Conclusions The trajectory of social inequalities, which may reflect the cumulative effect of family socioeconomic status, is a precursor of inequalities in adulthood.