In case anyone else is having to contend with a prolapsing stoma, I've found that what works for my son (8 years old) is to wrap an elastic belt around his waist. The elastic belt we use is about 3 inches wide and it fits directly over the stoma over the bag. It's been about a month now and he hasn't experienced another prolapse. Despite the fact that the stoma is covered by this elastic belt, it is still able to drain normally into the bag. My son wears the belt all day long, he doesn't wear it to bed.
Son 8 yrs old UC pancolitis, colectomy 7/07, no j-pouch constructed yet, sphincter muscles and part of rectum left intact so that j-pouch is a future option.