My 8yo adhd-c kiddo isn't on meds, but is hypoglycemic, so he eats every 2-3 hours. He maybe consumes 3-4000 calories a day. Unless he's in a growth spurt, and then it's double. But his schedule, quick like, is about 4 hours a day of sit down, and 8 of heavy activity. (Meaning either full speed, or constant movement, or strength schtuff. He has no problem jogging 3 miles in under half an hour, or snowboarding -tricks and all- for hours on end, or swimming for 3 hours straight -but 8+ hours of swimming total, etc.) Aka, he's using his body.
HOWEVER
My 8yo nephew is on meds (adhd-h), and a type where they go off of meds for the summer and other school breaks. We just had him for a 10 days, and even though my own son eats 2-3 times what "normal" kids do... this kid... OMG. 2 weeks worth of food in 5 days. I had to borrow against the next pay period's grocery money in order to keep food in him. I know how stretchy the alimentary system is, but he's a skinny (wiry) kid. Just WHERE (physically, not storage) the food got placed I don't even know. I'm dead serious he ate 3lbs of chicken strips I baked in 2 hours. That's over 4 chicken breasts!!! A whole box of cereal, and a half gallon of milk in the morning. Another half gallon that afternoon. 5 lbs of strawberries in a day. That was day one, and NOT including the 5 meals a day that I always serve.
His mum says it's normal for med-vacations. So I bit the bullet in the heat and switched to cooking (ugh) "winter food". High in proteins, fats, & vitamins... with a few heavy carbs (like mashed potatoes with chicken stock and butter, or rissotto, or "good" bread -like Grand Central Bakery thick crusty bread, as opposed to cellophane bread) as the "carbs" option. Nixed cereal entirely, and stuck with eggs & bacon & sausage & waffles for breakfast. HEAVY meals. And a constant veggie tray out, with a big thing of fatty ranch. And cheap popcorn smothered in nutritional yeast (for B vits), and gallons of greek yogurt with honey. Seriously, it was the American Heart Association Nightmare Diet 'round these parts. For several days we had bacon with every meal. Heart attack time. Oy.
BUT
After a day of "winter" food... he slowed down. It was pretty obvious his body was screaming for calories and vitamins & minerals equal to fats and proteins. As soon as I started having "dense" foods available 24/7 (we always have food available... since kiddo's hypoglycemic), he started eating less in volume. Which was a relief for my pocket book. If I could have found winter ration MRE's I would have bought them (6,000 calories per meal), but with the desert warfare being the norm these days, they're harder to come by.
I also broke into my husbands' weight lifting schtuff. He uses protein powder, peanut butter, and icecream to make a nauseating shake that's about 4000 calories. One of those a day helped my nephew out tremendously.
He put on only 1lb (his ped wants him 15lbs or 10% higher in body fat... he's only 4% body fat), so we didn't get as much weight on him as my SIL & I hoped) but it did stop him of "dyin' o' the' hunger"