Never force feed children. Thinness could be due to their genes. My suggestion is if they are healthy and they play- let them be. Just make sure they dont eat junk food (burgers pizzas etc) all the time
food .. Like peanut laddu, banana shake, French fries, dry fruit chikki , cheese sandwiches, cakes ice creams.. R all time kids favourite.. N helpful to weight gain too..
I hope you want to make them strong and not fat. Best to give them things that are rich in iron such as ragi and amaranth