My 2.5yo boy stays dry as long as I leave him bottomless. I speculate the issue may be getting to the potty and negotiating the underwear in time. If he's butt-naked, that streamlines the process enough for him to make it. Or, it may be that he's conditioned to wet his pants if he's got anything on below-waist. So far, I have been unsuccessful putting him to bed in his undies. We worked on wearing underwear around the house today, but he still ended up naked by the end of the day after going through three or four pair. I wish I had sound advice, but it seems I've felt my way through every potty training process so far. Waking her to go to the toilet seems the only real solution for keeping the bed dry, but not a fun one.