In the Healthy Sleep Habits, Happy Child book by Dr. Weisbluth, the author suggests that sometimes this happens when the child has a sleep disorder. Such heavy sleeping can be a sign of breathing problems at night. You might want to go to an ENT for an eval of the tonsils. I remember that the book said that sometimes having the tonsils come out makes the problem dissapear right away. I'm not usually a fan of such dramatic intervention, but I wanted you to have this info. My daughter, now 8, is a very heavy sleeper and had a big problem w/ bedwetting. We just kept her in a big kid night time pullup for years. Then last year, we went to a urologist. We tried the "try for dry" program (with a few modifications for us) and it worked great. The basic components are 1) an alarm, 2) a fiber supplement like Miralax, 3) a prescription muscle relaxant, and 4) dietary changes. The alarm worked for us, we used a Malem brand. It didn't wake her all by herself for the first week, but it was so loud that it woke me up and I went and woke her up. The theory behind the miralax is that if you aren't regular, fecal matter in the colon can put pressure on the bladder, reducing bladder capacity and making you need to urinate more frequently. This really helped for us, even though I was super skeptical at first. In fact, after she was completely trained, I stopped giving her the Miralax and noticed that she had a few accidents when she didn't have it. We skipped the prescription muscle relaxant -- this seemed a bit over the top for us -- but the theory was the same. Basically, the bladder is tiny and if it is relaxed a bit capacity can be increased. I'm not a big believer in drugs for kids, and I didn't feel that getting dry was this important, so we didn't do it. Also, I felt that if we made many big changes at once, and saw some improvement, I wouldn't really know what was causing the improvement. I don't remember all of the dietary changes that were suggested because very few of them applied to us. One thing I remember was that they said there is a big connection between ice cream for dessert and night time wetting. My daughter doesn't care for ice cream, so she doesn't ever have it for dessert.
We did have her evaluated by an ENT, who said that her tonsils were large, but not gigantic. He was on the fence about taking them out, so we did not do it. My daughter has learned to be dry at night, but she has also become a sleepwalker. I feel that the two are related, so we may have to revisit the ENT or have some sort of sleep eval in the future. Good luck!