Short answer: glutein allergy
The child could have allergy to some particular food.
Usually Milk or wheat, bournvita, or cereals, or pulses.
I recommend a liquid diet
with isabgol husk (non medicated) along with a diet of curds and srikhand .
She can be given sprouted moong dal about 25 gms every day in morning as it contains plant steroids
which are helpful to treat her present symptoms.
In the evenings at about 4 pm always give her an apple which helps to control diarrhea