What is mean BMI or table for average weight by height for children?

Tables online. There are many online bmi calculators you can download, entering the child's age, gender, weight and height. 5th-85th %ile is the range of normal. 25th-75th %ile is average. 85th-95th%ile is overweight. > 95th%ile is obese. <5th %ile is underweight. See http://pediatrics.About.Com/cs/usefultools/l/bl_bmi_calc.Htm.