Stop the milk immediately. Cats, and especially kittens, should not be drinking milk.
She should be drinking water. Try feeding her boiled white rice with minced chicken for a few days;.
However, as BOO says if she has diarrhoea for any period of time she should be taken to a vet as soon as possible. It takes no time at all for a kitten to become dehydrated and very ill which can even lead to death quite quickly.
Sorry if this sounds harsh. I sincerely hope she recovers quickly.