Cats, unlike dogs, are true carnivores which means they thrive on meat only diets and require no vegetable, grain, or dairy supplementation. Cows milk is not recommended for cats because it can be too rich for their digestive systems. This is because most cats can't digest milk properly because it contains lactose and most cats are lactose intolerant. The proteins in cow milk are too large for cats to properly digest which can cause intestinal upset, diarrhea, and vomiting.
Kittens especially can have difficulty digesting milk. There are other things that you can provide for your cat which are healthier. For young kittens that have been separated from their mothers before the age of weaning, try kitten milk subsititute. Recently, special new lactose-free milk has been developed and is now sold at pet food stores and most large pet supply stores. It is formulated to contain the same vitamins as the mother's milk and is gentle on kittens' digestive systems. These specially formulated kitty "milks" do not have cow's milk in them and are treated with the enzyme lactase to break down the milk sugar and make it digestible. Please consider looking for one of these the next time you would like to treat your kitty with milk.
Not all cats react badly to milk, but it is better be on the safe side. Generally speaking, milk is not recommended for cats, however, some experts advise that cream is better than regular milk if you must give it to your cat, most likely because cream has less lactose than whole or skim milk.
Essential to a cat's health is plenty of clean water. Change your cat's water dish regularly and keep it within easy reach.