Just curious, no other answer seems to be specific. I'm not looking for a discussion here, just an answer and an explanation.
I would say no, cereal in milk is more simply comparable to a food in sauce - there's very little interaction between the cereal and the milk to make it a single dish, it is not a cohesive whole - and in fact it is considered the same dish (cereal) if eaten dry. It might work as a simple sweet pudding if allowed to soak and meld a bit, perhaps, instead of being eaten crunchy (which is where I was getting the food-in-sauce likeness).
Hot cereals might count a (sweet) soup, especially if there are other ingredients (like a gruel with grain and milk, and spices, dried fruits, and egg yolks to thicken) - though I think it would depend on consistency. Something thicker, like a gummy oatmeal, would probably be more closely related to a pudding, but a thin one might well be soup. Even cold cereal might work as a (sweet) soup if there was more going on with the milk, some other ingredients and flavorings - then it would be a cold, sweet milk soup with the cereal bits acting like dumplings.
The dictionary definition of soup is basically a savory definition, citing a combination of liquids, or meat and vegetables in broth or water (with other ingredients for flavor and texture), though there is a secondary definition that covers mixtures or substances resembling soup - which is how sweet soups, cold soups, and other more unusual sorts of soups earn the name.
I think (cold) cereal is just too far removed to earn that label, being cold instead of hot and sweet instead of savory and separable into its parts instead of married into a single dish and only having two ingredients, that is, not being a complex mixture or a greater-than-its-parts whole, and having other words which are better definition matches (cereals, puddings, foods with sauce, etc). A soup can still be a soup with a couple of these alterations, I think having all of them is too many for it to fit that category.
Cereal in milk is not soup. From Wikipedia :
Soup is a primarily liquid food, generally served warm or hot (but may be cool or cold), that is made by combining ingredients such as meat and vegetables with stock, juice, water, or another liquid. Hot soups are additionally characterized by boiling solid ingredients in liquids in a pot until the flavors are extracted, forming a broth.
Traditionally, soups are classified into two main groups: clear soups and thick soups. The established French classifications of clear soups are bouillon and consommé. Thick soups are classified depending upon the type of thickening agent used: purées are vegetable soups thickened with starch; bisques are made from puréed shellfish or vegetables thickened with cream; cream soups may be thickened with béchamel sauce; and veloutés are thickened with eggs, butter, and cream. Other ingredients commonly used to thicken soups and broths include egg, rice, lentils, flour, and grains; many popular soups also include carrots and potatoes.
There is much more information on the linked page but I can't see any way that cereal in milk would be classed as a soup.