I'm so sorry to hear this!
For diarrhea, I can really recommend to give them no food at all for 12 hrs. In many cases this will be enough- if not go for 24 hrs without food.
I can see that this may not be ok for the babies but definitely for the grown-ups. It helps their intestines to calm down.
I don't believe in dry food too much as it is mainly based on cereals which is hard for a cat to digest.
I hope very much you get them back to health and happiness soon.