J.R.
Hi D.,
When my daughter was 30 months I trained her to go potty. We would make a big deal when we heard it hit the water and she would just beam - this was long after the peanut m&m's were gone. If I was going out, I would tell her if she wanted to come along she would have to go potty first.
We kept her in her pretty panties and put her to bed the first day she'd gone without an accident - which was the first day she was trained - and she was fine overnight. Now I know all kids are different, but she's never had an accident in bed. She has had accidents when she would get too busy playing with other kids to get to the potty in time.
Hope this helps. I'd put a waterproof cover on the mattress (they have plastic ones you can zip around the entire mattress) and give it a shot. J.