Background: South Asians are prone to cardiometabolic disease at lower BMI levels than most other ethnic groups, starting in childhood. The magnitude of BMI misclassifications is unknown. Aim: To compare the BMI distribution of contemporary South Asian 0–20 year olds in the Netherlands with: (1) The South Asian norm reference (secular trends); and (2) The WHO child growth standard and reference. Subjects and methods: The BMI-for-age distribution of 6677 routine measurements of 3322 South Asian children, aged 0–20 years, was described with the LMS method and BMI z-scores. Results: The BMI distribution in South Asian 0–4 year olds was almost similar to the norm reference (mean BMI z-score = 0.11, skewness = 0.31, SD = 1.0), whereas in 5–19 year olds the distribution had shifted upwards (mean = 0.53) and widened (skewness = −0.12, SD = 1.08). Overweight (incl. obesity) and obesity peaked at 8–10 years, at 45–48% and 35–37%, respectively. Relative to the WHO references, the BMI distribution was left-shifted at ages 0–4 years (mean BMI z-score = −0.46, skewness = 0.23, SD = 0.98) and widened at ages 5–20 years (mean = 0.05; skewness = −0.02, SD = 1.40). At most ages, thinness rates were significantly higher and obesity rates lower than based on South Asian norms. Conclusions: A secular change of BMI-for-age in South Asian children mostly affected children >4 years. WHO references likely under-estimate overweight and obesity rates in South Asian children.