Kittens after 5-6 weeks of age (which is when their mom weans them off of her milk) don't require ANY milk. Cats and dogs lack the enzyme to properly digest the milk sugar (lactose) found in cow's milk. Esentially they are lactose intolerant. They can get very bad diarrhea. After kittens have been weaned from their mother, they can go on regular kitten food until roughly a year old, then they can go on adult cat food. If the kittens have trouble chewing the kitten food, you can soften the kibble with warm water (let it sit for a few minutes so the kibble can absorb the water) and make it easier for the kittens to chew. Rest assured.....if they still required milk.....their mom would still be nursing them!
They are kittens, what is wrong with "Mother Cats" Milk???
5 to 6 weeks is alright. Or wait till the mother starts to ween them. You can give them water anytime.