I know something that my parents recommend and seems to work for my other siblings whos kids wet the beds, is to try to wake them up in the middle of their sleep and have them go to the bathroom then. That way they have the chance of sleeping for a while, then bathroom then they can go again when they wake up.
My daughter whos potty trained has only wet the bed twice in the 2 or so months that she's been out of the pullups. Both were when she was sick and slept longer then normal. What I'm guessing would be a good expansion of waking them up, would be to take note of how long they can hold it in when awake and then maybe figure how long from when they sleep to when they'd need the bathroom would be. (so if its 7 hours, if htey sleep at 8 have them use the bathroom at 2:45 or so). It might take a few times to test around with when to wake them up to use the bathroom, but I heard it works great!