Some of the simple things that can cause complaints of stomachache in children are constipation and overeating. Some children hold in their bowel movements all day, either because they are too busy to go or they don't want to use the bathroom at school. This can cause some true discomfort by the end of the day. Similarly, the child who gets a stomachache after lunch every day may be eating too much and overdistending the stomach.
If a child has no other symptoms besides the pain it can be difficult to sort out the cause. One thing that I often have patients do is to keep a "diary" of the pain. Write down when the pain occurs, what part of the abdomen it was located in (high up or low down, right or left side, or all over) , how long it lasts, what was going on at the time, what relationship it had to eating, etc. By looking at this type of diary over a week or two, it is sometimes possible to determine the cause, or at least the associated factors. Once you've generated a diary like this, you can review it with your child's physician, and you can both play detective as you figure out the associations. Things such as lactose intolerance might not be suspected without recognizing the association of the symptoms with the use of milk products. A child whose pain only occurs at bedtime may have some nighttime fears or other anxieties.
One general guideline with assessing the significance of stomachaches is to determine if they interfere with the child's activities. A child who complains of a stomachache and then wants to go out and play 10 minutes later is less likely to have a serious disorder. A child who complains of pain and doesn't want to go to a birthday party or a special outing is more concerning.
There are some symptoms that should make you more concerned about stomachaches, and if these occur, further evaluation should be done: Chronic diarrhea, persistent vomiting, fever, blood in the stools, burning with urination, abnormal menstrual periods, and weight loss might indicate more serious disorders.