Most cats are lactose intolerant, and can't digest the lactose in cow's milk. So the kitten should never be given cow's milk. Kittens' digestion is very delicate, and they will get diarrhea very easily and dehydrate. read more
You can also feed them kitten milk replacement formula. Cow’s milk can upset a kitten’s stomach and should be used as a last resort. Kittens should be drinking water by the time they are 4 to 6 weeks of age. read more