Body Mass Index (BMI) is used to assess whether or not children are considered obese, overweight,underweight. It is a number calculated from

BMI percentile

For children, BMI is plotted on a growth chart that uses percentile lines to tell whether a child is underweight, healthy weight, overweight, or obese. Percentiles are the most commonly used indicator to assess the size and growth patterns of individual children in the United States. The percentile indicates the relative position of the child's BMI number among children of the same sex and age.
A child whose BMI below the 5th percentile is considered underweight. A child is equal to or greater than the 5th percentile and less than the 85th percentile is considered a healthy weight. A child at or above the 85th percentile but less than the 95th percentile for age is considered overweight. A child at or above the 95th percentile is considered obese.