It depends on which resort you are staying at and how hot/tired you (or the small child) might be when returning. Generally, during the day, monorails departing Magic Kingdom are less crowded with little/no wait since most people are either still arriving or at the park. Other posters have mentioned the various transportation options, monorail runs in one direction with set stops. The Contemporary, for example, is the first monorail stop after Magic Kingdom, compared to Floridian with is the last stop before Magic Kingdom. Monorail allows you to sit and has AC and stops inside the Contemporary. There is also a walkway between Magic Kingdom and Contemporary. With all 3 resorts you still have to walk to your room once you depart the monorail. We were just at Disney last month and found convenience/comfort to be more of a deciding factor rather than whether it takes 5 or 20 minutes to get to your destination.
Depending where you are in the park when you decide to leave for a nap, there is also the time it will take to walk to the park exit.