Kids who have this kind of discomfort after consuming dairy products might have lactose intolerance, which is caused by problems digesting lactose, the main sugar in milk and milk products.
Lactose intolerance happens when the body makes too little of the enzyme lactase, which is needed to break down lactose into two smaller sugars, called glucose and galactose. When there's not enough lactase in the body, lactose doesn't get broken down in the small intestine, and it passes into the large intestine where bacteria ferment it into gases and acids.
This process can cause cramps, abdominal pain, gas, and diarrhea about 30 minutes to 2 hours after consuming any foods or drinks that contain lactose.
http://kidshealth.org/en/parents/lactose.html?WT.ac=p-ra
