Hi M.!
I was quite curious about kegel exercises and children. So I did some research and found nothing! There have been numerous studies about kegel exercises and bladder training. However, all pertain to older women in their 40s and 50s. I did find that 20% of children over 4 wet the bed(www.seekwellness. com/incontinence/children_and_bladders.htm).
This website is maintained by a medical center who specializes in bladder function: http://www.chop.edu/ consumer/jsp/division/generic.jsp?id=84988. I think you will find it useful. They use kegel type exercises for treatment, but they use computers as well. However, they have several different types of treatment. They decide which would work best after a full evaluation.
In my opinion, stepmom needs to back off. We all know that babies and children respond best to consistency. If you can't do what your ex and his wife do because of schedules and such, then it should be scrapped. Why is she so gung-ho?
I'm not sure what your ex's wife does for a living, but I would STRONGLY refrain from having her and your ex teach your daughter kegels. When things aren't done or explained properly, it can have long term effects on a child. I've seen it at the psychology office I worked. If kegel exercises are something you would like to try, ask your pediatrician to explain it and show your daughter. Or have your pediatrcian tell you how to go about teaching your daughter.
Night-time training is even harder than day-time training. All children learn at different paces. You can't force the issue. It can damage your daughter's self image and self esteem. You have some great advice here. Maybe you and your ex can agree to something else here.