J.M.
I'd say: Why do they feel the need to be potty trained at night when they are wearing a diaper?
I'd take 'em out of the diapers. My oldest son had a number of night-time accidents at first. His matress is covered in plastic and he only had a small pillow (not the standard twin sized one - because they take MUCH longer to wash and dry!) I had a "back-up bed" set up for him - with clean sheets and pjs. I also kept the heater turned up a bit at night - so he didn't need more than a sheet and light blanket comforter to stay warm. When he wet the bed in the middle of the night, he would get out of the wet things and settle himself into the back-up bed without waking me up. In the morning he would help me strip the bed and put the soiled items in the washing machine. The accidents got fewer and fewer - and now he wakes up anytime he needs to go to the bathroom no matter what time it is!