The worst mistake you could make is to offer your child treats instead of food out of fear that your child will starve if he doesn't eat. Once you give a child with a small appetite junk food in place of meals, since his appetite is small, he will just continue to refuse food until he gets the treats he wants. That may provide calories, but it isn't nutritious. Even overweight kids can be malnourished if they live on junk. Some foods that are usually thought of as junk food can still provide your children with the basic nutrients.
Hot dogs are high in fat and salt.
I wouldn't feed them to my child every day, but they do have meat protein in them. Same for hamburgers from fast food places. Pizza is an excellent source of nutrition. The cheese provides essential protein and fat while the crust provides carbohydrate calories. Maybe you can even sneak some vegetables onto the pizza.