This Body Mass Index (BMI) percentile calculator from the CDC is designed for children from ages 2 to 19. It is based on the birth date of the child and the date the measurements are taken.
The CDC emphasizes that this is an approximation, and that you should see a medical professional to obtain accurate measurements of body fat.
The site include a section defining BMI, how the calculator is designed, and BMI percentile tables for boys and girls. In addition, there are tips for parents. Another section includes an Excel spreadsheet tool for schools.