You know how kids like to have a choice? Like to be in charge? If you are fighting with your kids about nap times, here is a funny suggestion.
My mom told me that when we were little and fighting her about naps she would ask us a simple question. "Do you want to take a long nap or a short nap? --- It's your choice!" she would say. If we complained and whined then we had to take the long nap. Of course we always wanted to take the short nap.