In children, a small amount of blood in the stool is usually not serious. The most common causes are constipation and milk allergies. But it is still worth reporting to your doctor, even if no workup is necessary.
Blood in the stool may come from anywhere along your digestive tract, from mouth to anus. It may be present in such small amounts that you cannot actually see it, but is only detectable by a fecal occult blood test. When there IS enough blood to change the appearance of your stools, the doctor will want to know the exact color to help find the site of bleeding. To make a diagnosis, your doctor may use endoscopy or special x-ray studies.
If it's bright red, it's coming from the lower colon or rectum. It could be a small tear in her rectum from constipation. However, it could be multiple other GI problems that need a doctor's intervention. Ask her if she is straining when she goes to the bathroom. Increase her fiber and definitely water. It's a habit that she should get rectified (sorry) so she has good bowel habits.