Have you tried offering them any of the milk products specifically designed for cats? I've seen a powdered goat milk with added probiotics at my local pet store, there's formula replacement for nursing kittens, and at least one other that I've seen.
Edit: Chewy seems to have quite a few "milk" products that are intended for cats: https://www.chewy.com/s?query=Milk&rh=PetType%3ACat