We let our kids eat when they're hungry. I would do things like cook extra sausages or meat patties, or maybe some roast meat, and keep it all in the fridge with fresh fruit and vegetables. The kids know they can help themselves (just let me know if they eat the last of something). That way I know they are eating healthy nourishing food.
Our rule is - clean your teeth before you go to bed. If you want to eat after you've cleaned your teeth that's OK, but you will need to clean your teeth again.
I do think a lot of this is the stims.
difficult child 3 was very late taking his medications on Tuesday (because I was so sleepy, after not having got to bed until 4 am). He kept bugging me for food, was constantly eating. He ate a big bowl of muesli and was still ravenous. I gave him a plate of roast lamb and then I cooked several big bowls of popcorn for him, he was snacking all morning.
I've found if they go to bed hungry, they get up earlier and are more cranky too. I try to keep them fed as much as possible, to make my life more pleasant. And they're still skinny.
Marg