when I was a kid, Good Housekeeping used to run ads from some company or other that stated that 1 out of 9 kids wets the bed at age 9.
Which, however accurate or inaccurate (it was in an ad, after all), has been useful in helping my kids: "so, at least one and probably two other kids in your class probably also have this difficulty" (and via parent grapevine I know who one is for sure, so I can say this with confidence ;) )
so ... I have heard over the years the too-small bladder, the immature bladder, a bunch of stuff which doesn't seem to really explain at least two of my kids (although undoubtably correct for many cases) ... the chiropractic/nerve pinch answer I just sent a flower to, because when I injured my lower back in a car accident, *I* had bladder sensing and bladder control problems (SIGH!!).
However, about a year ago(?) I read that someone somewhere had figured out that there is a hormone/sleep situation (I can't even remember if it is a hormone that is supposed to turn off during sleep so your body stops making pee, or a hormone that turns on during sleep to actively suppress making pee) that does match well my kids' symptoms, and is very logically an inheritable pattern.
Although I have to say that I don't remember any kids in my family, and I haven't heard of any kids in my Ex's family, having this problem ... must've been a double recessive or something ;).
Life got so much better when I admitted what was clear--the kid(s) didn't want to pee the bed, the kids don't LIKE to pee the bed, and the kids ... had no control over it.
Diapers are my friends. They still leak through sometimes, but the amount of bedding is a lot less. Liberal application of BacOut (an enzyme cleaner) covered with cloth diaper(s), covered with a towel, a sheet/underblanket ... kid back to bed (never having fully woken up, if I'm lucky). Have extra blankets (see also, Goodwill ;) ), especially easy wash-and-dry fleece blankets. I gave up on tucked in sheets--and pajamas-- a long time ago. (Hard to deal with at 3 AM wet and stinky? buh bye.)
Anyhow, hope all these reports from the trenches help :).