It is not unusual for kids to have an upset stomach now and then. Often, they just will have eaten too much sugar or too large a meal, or their stomach is reacting to nervousness that they don't quite know how to express. Sometimes, though, an upset stomach is representative of a bigger issue that does need treatment by a pediatrician. How do you know the difference? Well, you should call the doctor if any of the following are true of your child's upset stomach.