I would be interested in what sort of doctors you have gone to see. Have you been to a doctor that specializes in kidney and bladder functions? If you haven't seen a urologist I would ask for a referral and speak to one of those. Just because a doctor SAYS that nothing is wrong with her doesn't mean there isn't, he may just not be specialized in it.
Another idea though it sounds strange is to take her to a chiropractor. Kids spines get out of alignment just like adults and nerves can be pinched that control bodily functions like bladder control. My chiropractor adjusted my son's lower back and we haven't had an accident since. My chiropractor told me that many kids go to him and he has had great success with it... its worth a try.