My grandson is 9 and has a appetite similar to your dgs.
He is growing upwards as we watch! I can hardly believe how tall he's gotten in just 6 months.
My sister, thin as a rake, used to eat constantly - a box of Weetabix every couple of days in addition to her 3-4 regular big meals and often taking a cake to bed with her.
She freely admitted she never, ever felt full.
I who was on a strict diet (no sweets, fried foods, cake etc) because I was fat from birth, constantly put on weight, eating a fraction of what everyone else ate.
There's no justice but at least now I know why it happens.
Incidentally, I have been labeled 'obese', better yet, 'morbidly obese' most of my life but it has in no way restricted me from having an extremely active life.
Even the health problems which have come with my 70's are nothing to do with being fat.
So let's not run away with the idea - much outdated - that being fat means early death and useless to society.