I looked up the definition of cereal and soup, and found this:
Cereal: A breakfast food made from roasted grain, typically eaten with milk.
Soup: A liquid dish, typically made by boiling meat, fish, or vegetables, etc., in stock or water.
Since I have no idea what stock means (in the soup definition), I looked that up as well: liquid made by cooking bones, meat, fish, or vegetables slowly in water, used as a basis for the preparation of soup, gravy, or sauces.
I think we can agree that a cereal is not a soup.