My 2 youngest kids are skinny. One is only 5% on the weight table. My son is not even on it.
My kids are not picky eaters though. I offer a lot of dense calories foods for them to eat.
Breakfast always has bacon, sausage or eggs with ham and cheese, with whole grain breads or cereals or pancakes.
I make steaks, mashed potatoes with sour cream and butter, Caesar salad. They love Korean meats, like kalbi and bulgogi.
Lunches are pastas: Mac & Cheese, raviolis, bread and butter, salami, fresh fruit, chocolate chip cookies....
We eat a lot of fresh fruit and veggies.
If your kids are active, doing well academically, sleeping well, I do not recommend making them 'finishing their plates.' My parents made us do that and I hated it. Some foods are just gross to me b/c of that approach.
Some people's metabolisms are just higher and they burn through it faster. Also, is there anyone on either side of your families that are built thin and wiry?