Cow's milk is not the best for kittens. Ask your vet what to do. Cow's milk won't hurt kittens, so you can use it if you have to. It should be body temperature.
When they are young, under about 8 weeks old, it is what they naturally have as their food source. However, only cat milk. Kittens are weaned at about 8-10 weeks and milk becomes hard for them to digest when they are adults.
