The temperature of the milk does not affect the nutritional content of the cereal. The only way milk affects the health of the person eating it is if the person ate the cereal with spoiled milk. Then the person would get sick. Even if the person did eat the cereal with spoiled milk, they may get sick but they will still get the nutrition from the cereal.
It doesn’t matter whether you eat cereals with cold or warm milk because it keep the nutrition the same. Warm milk does not harm the cereal’s nutritional content. Some cereals are meant to be eaten warm like cream of wheat or oatmeal. Cream of wheat is not meant to be eaten cold and it contains a lot of nutritional value.
Considering that in this country milk has been pasteurized, which is a process where the milk his heated to elevated temperatures to kill bacteria and give it a longer shelf life. During pasteurization some of the nutrients are lost due to the elevated temperature.
Also, most cereals have questionable value for good vitamin enrichment. If your milk is warm when you put it on your cereal, aside from not being as good as cold milk there is no change in the nutritional content of the cereal itself.