Most cats are lactose intolerant! Please keep that in mind, especially if you just adopted a stray kitten.
Kittens need milk from their mother in order to survive. But, if they aren't together, avoid feeding them with any milk with lactose. You should try soya milk, since it's lactose-free! It's one good alternative. There's also a "cat milk" available in the supermarket. You should ask a vet about it first! ;)