You know how in preschool, the kids are supposed to take a nap, but sometimes some kids don't want to? This is how I would fix the problem. I would go up to them and say, "Listen, in 20 years you are going to wish someone came by after lunch and made you take a nap. Enjoy it while you can!" I think the kids would not longer want to stay up. Instead, they would want to cry because a strange man had just yelled at them. Problem solved.