[sbaglio]

Storm, lactose intolerance in children is not very common (congenital lactose intolerance is extremely rare). Usually a child will have some sort of GI illness, which causes the GI tract to slough off the lactase enzyme, making them intolerant of lactose until the bowel heals.

In reality, not many children are put through the recommended hydrogen breath test for a diagnosis (I'm not sure if the test is as sensitive in kids...). One simply cuts out all dairy from the diet for several days, then reintroduces it, and monitors the symptoms. If there are no co-morbid conditions like inflammatory bowel disease or IBS to cloud the picture, the diagnosis can be pretty clear if one takes these steps.

Celiac disease (gluten sensitivity) is often underdiagnosed in kids, so it's something else to consider when there are symptoms that resemble lactose intolerance.