i think hypno would only work if the root cause of the bedwetting was psychosomatic/behavioral etc. I don't believe hypnosis would cure an overactive bladder.
Re use of hypnosis - I'd give it a try no matter what, as I've found it to be helpful for many things. If you go to a trained hypnotist, be sure and get a CD or tape of the session. It is important to reinforce the suggestions at least once a day for at least a month. You can't go and be hypnotized just once and expect changes to take effect. Reinforcement is KEY.