I was over weight as a child and my Mom helped me out with a diet plan when I was about 11 I followed it and dropped the weight with just a little activiaty after that I became more active on my own. I actually started doing things like buns of steel when I was 12 I have always had to work at it to stay in shape. Teach her good eating habits first as well as being active focus more on things like light weight lifting and cardio. I did start lifting heavier weights at 15 when I first joined a gym.