You don't need to bring milk. They have milk at breakfast time.

We usually brought some formula and bottle water just in case. And with small kids, we always get cabin with balcony since we will spend more time in the cabin (when kids take nap..). To us, we perfer Princess, but if money is issue, then use Carnival. Princess has better food. Also you need to check which cruise line has free kids center for 2 years old. As I knew, most are for 3+. We had to take care of our 2 years old ourself.