Conventional BMI calculators don't work for children, so I thought you might be interested in this link, which allows you to calculate your child's BMI by including factors for sex and age.
www.keepkidshealthy.com/welcome/bmicalculator.html?ye ars=NaN&months=NaN&gender=NaN&feet=NaN&inche s=NaN&inchespercent=NaN£s100=NaN£s10=NaN& pound;s=NaN£spercent=NaN&start=1&page=2