Re: bowel problem
What doctors have you been to? Have you been to a pediatric gastrologist? Sometimes children develop a megacolon. They have held the BM--maybe because of fear that it would hurt. Gradually the colon because distended and they don't really feel the urge to go so the fecal matter leaks out.
Do a search on encopresis as this is what soiling is called. There is a whole retraining program of the bowel that involves the use of fiber supplements, high fiber diets, and going to the bathroom at the same time everyday--usually after a meal. This should be directed by a doctor or other professional.
Has she had this most of her life or is it recent? If it is recent, I would also investigate what is going on with her emotionally and if there is any trauma in her life that you don't know about.
Doctors should be guiding you in getting some help for this. If you have a Children's Hospital in the area, then get a referral there.
As far as sitting in it, I think that children become immune to the smell. Is she resistive to changing and cleaning herself when you talk with her? Be careful about being negative, she already feels bad about it even if she is not showing it. That will not help it. Handle it as matter of factly as you can. It is frustrating but there is help out there for it.
Let me know if you have any questions.