My niece has disordered eating, 8 yrs old?

My sister has always tried to make sure her kids eat healthy and are active. At one point a few years ago the doctor actually told her she was a bit underweight, she was 6. My sister became much less restrictive with what they ate. But now her daughter is obsessed with what she eats and her weight. Everytime she eats dinner (because she's forced to) she immediately gets up and literally runs around the house to burn the calories. Sometimes she asks for dessert (which we thought was progress) but then she told us it was so she could have more energy to run around, and everytime we gave her dessert she would run around so much she would vomit. So now we don't give her the dessert anymore. It seems like a form of bulimia she's created. There are no scaled in the house but the school nurse calledafter she was visiting her everyday to use the scale. She goes to a therapist and we've seen a psychiatrist but we believe she's too young for medication. Were at our wits end and really don't know what to do. We live in massachusetts and were looking at places for her to go into a resedential program, but everytime we visit all the girls are much older then her and she wouldn't feel comforotable. Please help!