Childhood obesity is defined according to curves generated by the World Health Organization. It's a little bit complicated. It's actually reasoning backward or extrapolating backward from those BMI criteria that in adulthood—say, above the age of 19 years—are associated with an increased risk of chronic diseases and higher rates of mortality. Reasoning backward or extrapolating backward from those curve points, “overweight” in adults is defined as a BMI above 25, or 25 to 30, and “obesity” is above 30.
Reasoning backwards on these growth curves, we can see that if a child's above the 85th percentile for their BMI at a given age, they're categorized as overweight. If they're above the 97th percentile, they're categorized as being obese.