My cousin has a child also who will eat and eat and eat and eat and eat and eat and still say he is hungry, is morbidly obese because of his eating, and also has ADHD, as well as other issues, including obsessive-compulsive disorder, although he is able to function normally when he wants something. He was a preemie that at the time he was born was on the border of where the docs are able to keep them alive. His issues have gotten worse and worse as he has gotten older, to the point that as a young teen my cousin was unable to handle him, violence was even threatened. He needs an extremely structured environment and has had to be placed in a facility dealing with teens with eating disorders, behavioral problems, etc. He came home for a few years, did really well at first, but has recently been placed back in such a facility. Frankly, I don't know what she is going to do because he will be 18 soon.
I write this not to frighten you but to reinforce the advice given to you regarding talking to your doctor. My cousin and my aunt, mostly my aunt, his grandmother, kept saying for years it's just a phase, he'll grow out of it, he's just being a boy, etc. Well, he didn't grow out of it and it's only gotten worse. Please take them to see their doctor.
With the above said though, my grand-daughters are 4 and 8. For the last few years, the 8-year-old has been going thru periods where she can eat third helpings and be hungry again shortly after. This will go on for several weeks and then taper off for a few months, then start again. She's tall for her age and has a good body weight for her height. One of those cycles is just ending. When I was there about a month ago, she was eating everything she could get her hands on (and getting up in the middle of the night and eating - driving her mother nuts), yet when I went this past weekend, she is nearly back to eating normal amounts. It's obviously a growth phase she is going thru so please consider that as well.
A month ago when I was there, I talked to her about eating in the middle of the night because it was driving her mother and father nuts since she was taking food in her bedroom and leaving wrappers, crumbs, etc that were drawing bugs (South Florida too just several hours away from us). She was defensive at first because her mother and father had been fussing at her so much and punishing her over it. Once I got her to realize I wasn't going to fuss at her, she told me she couldn't help it, that she'd wake up so hungry she couldn't go back to sleep and she just had to have something to eat. The younger one would usually wake up and tag along on the kitchen raids too.
So, please take the kids to the doctor and find out if there is a problem before fussing at them or punishing them.