Child body mass index, and about the childrens BMI charts in this article
And in the chart below, the blue line (boys) and pink line (girls) shows how the CDC’s 85th percentile line of Body Mass Index (overweight threshold) compares to the above data.
There was also an interesting chart showing the percentiles of body fat in children. Particularly interesting is the dip in body fat that occurs during puberty in boys.
A link to a fairly new and good CDC calculator of BMI percentile of children.