For children, how do you slow down metabolism without my child becoming an obese teenager, she's 11 years old?

my daughter is 11 years old. she's a picky eater. she's adhd and the new meds that she's been on have made her eat more, i had no idea she would gain so much weight, when she had lost so much, she's been on this medicine since last july (2010) she has gained around 66 pounds, some is her growing. I have noticed in the last few months that her metabolism is a high (or fast) anyhow, I want her metabolism to slow down, she doesn't need to gain anymore weight. we don't exercise that much, but i try to watch what she eats too. just an fyi, she know has a stimulant pill and a non stimulant in the afternoon. When she first started taking meds, she was on one pill and it was a stimulant and she dropped at least 10 pounds besides running in PE. so my question is this, how do you slow down a child's metabolism.