This is the one I used for my daughter. It gives the
BMI and I just kept adjusting the weight until she was in the 50th percentile and took the difference as the # of lbs she needed to lose.
http://www.kidsnutrition.org/bodycomp/bmiz2.html
I just tried to find something fun for her to do. So I taught her how to ride a bike and we ride together. It's great fun as we ride out by a park and we joke that we're going to pass each other and I joke that she's trying to run over the chipmunks.