Many times kids in the system struggle with food issues. They can range from anorexia to obesity. Some do not know when to stop eating and some cannot make themselves eat. And it all stems from years of not having enough or not knowing if you'll have enough.
Many scenarios could have taken place in the past to cause these issues. Many times, children have gone hungry, or there has just been a lot of uncertainty as to where the next meal might come from. Food stamps might have been traded for drugs, the house might not have any food it in, and the child is hungry. And so they either go hungry, or they have to find their next meal. Maybe a neighbor would feed them or they could find food in the trash bin at the fast food place down the street.
If it is a sibling group, the oldest child feels responsible to get food for the younger children. If the baby still needs milk in a bottle and is crying for it, it might be up to the 5 year old to find some milk. Either way, having enough food becomes a central issue. The survival instinct takes over and it becomes not only an important thing, but the ONLY important thing.
Fast forward ahead five years to when the child is placed in a safe, loving adoptive home where there is always enough food to eat and nobody ever grows hungry. There is breakfast served as soon as you get up, a morning snack if you aren't in school yet, lunch, an after school snack, supper, and evening a bedtime snack. The kids can look in the fridge and see that it has food. They can look in the freezer and see it stocked. They can see that the cubhoards have enough food to feed the entire family for weeks.
But even a year or two after they are placed you still find moldy cheese under their beds, cracker crumbs under the pillow, and chips shoved in backpacks. You still find a kid that doesn't know what it means to feel full and overeat until they are ill. You still have a kid who might overeat and then go make herself throw up.
Trying to behavior modify children from their food issues is practically impossible. You can consequence, take away, threaten, etc., but the issues are not going to go away. You cannot take away that basic instinct survival feeling from a child, no matter how hard you try.
Acceptance and supervision and gentle attachment will help it to slowly disappear. Maybe not completely, but the symptoms will lesson. Have patience, remember the whys, and help them heal, regardless of how long it takes.