N.S.
I know the tests sound like alot for a toddler, but its really important to do this. Diarrhea is serious business. Has the doctor checked for food allergies? People with gluten allergies, as well as other allergies often have chronic diarrhea as a symptom/result. And what about irritable bowel syndrome(Colitis) or Chron's disease? Has the doctor checked for these too? These types of disorders are hereditary and can go unnoticeable until one starts eating "trigger" foods.
One other possibility, but highly unlikely because you say she's had tons of blood tests etc that came back normal, but just in case you didn't know, there is a new strain of staph infection on the rise that affects the intestinal tract. This is a very serious form of MRSA, and from the recent article I read on www.abcnews.com, it often goes missed until the patient is severely ill because it is often mistaken for a severe stomach flu. I'm not certain, but I think this story ran about a month ago, and you might be able to pull it from their archives.
The story was about a woman who contracted it after delivering her baby in a particular hospital. For six weeks or more she had continuous diarreah. It wasn't until her electrolytes were totally messed up and she started having problems that doctors thought to test her for a bacterial infection and discovered she had MRSA. She's now being treated with super anti-biotics.
Hope this doesn't upset you, but it might help you think of things to talk with your doctor about.