Hi M.,
We went through the diarrhea illnesses this winter. The one thing I read and tried that seemed to work was eliminating dairy until everything is "normal" again. Apparently, when we get diarrhea, we can become sensitive to dairy until all of the healthy bacteria is in our intestines again. As I said, it seemed to work with my 3 and 5 year olds. With my daughter, it took 3-5 days. With my son, who is lactose intolerant to start out with, it took more than a week.
I fed them pretty much what they felt like eating.
Good luck.