Truly, it depends on the child, the age of the child and other factors.

If it's accompanied by other signs of the flu/illness then the parents get a call. Including: fever, unusual cough (I say unusual because my children sometimes cough as a result of asthma), lethargy, ect.

With very young toddlers or babies it can be because of excessive drooling/teething, new food, ect., and not be a bug. If this appears to be the case then I just deal.

I also agree that diarrhea in older children is more likely to be an illness and so I will call.