I know that with adhd (I am also the add/adhd CL), a child on medication is not hungry until the meds wear off and then they are very hungry. And many times this is after supper is over. Any chance this is medication related?
What time of the day is she sneaking food? and what kind of foods? The normal answer if they are growing. Let them eat. Just make sure that the only food they can get is good, healthy food.
The pain meds are different. First, you need to know if she has a reason. Second, what type of pain meds and how much ? Some are very dangerous and some are not in the right amounts. Must should be taken with food to prevent stomach problems. She is old enough to be made aware of the dangers (for any kind of pain med). Any pain med should be probably locked up.
Without more info, I really can't say much more.