The alarms are great for the right kid. I used it for my oldest daughter when she was three. She also had started out potty trained and NOT wetting the bed and then started. She did GREAT with it and was dry by night three and as long as she wore her alarm stayed dry. She had to wear it for confidence for a year. She will still wet the bed once in a while. (every six months or so)
My younger daughter did the same thing. She was potty trained for an entire year dry at night and then started wetting the bed. She was also three. I used the alarm on her, but it didn't even make her squirm. She was too heavy of a sleeper. On the third night of using it I went to wake her when the alarm was sounding and she in a groggy mostly still asleep voice said "I tried the best I could Mommy." I took it off. We haven't used it since. At this point my daughter hates wearing pull-ups to bed and wants to be dry. She is four now. So its time to try another alarm for us. I am thinking of getting the kind that vibrate, too. The one we have now just makes a chirping noise and has a flashing light. Doesn't work for her.
They actually recommend them for ages 6 and up. You can go to www.bedwettingstore.com and get some info. on them. I can assure you that none shock the kid. tee hee
Some times growth spurts will cuase bed wetting. But I know how you feeling having gone through that. When my younger started wetting the bed it was at nap time, too. I was so frusterated. I didn't understand why. But she was never a really sound sleeper until then. So maybe she is just sleeping so sound now that is what happens.