You don't need ice. Just use cold milk from the refrigerator, which is where milk is supposed to be anyway. Your cereal isn't going to be warm unless you put it in the microwave.
Are you talking about putting ice in your hot cereal (oatmeal) or putting ice in a cold breakfast cereal like Froot Loops or Frosted Flakes? If you chose the latter, wouldn't it make your milk watery and tasteless?