D.W.
We have several friends whose children (ages 5-6) still have nighttime accidents. It's for a variety of reasons, but they simply use pull-ups or something like that to help prevent the hassles of dealing with cleaning sheets daily, etc. In one case, the daughter is a heavy sleeper, and no matter what they do, she can't recognize when she needs to go to the bathroom in the middle of the night.
Here's what the American Academy of Pediatrics recommends:
http://www.healthychildren.org/English/ages-stages/toddle...