Does having polycythemia mean that my kid has cancer?

No. In general, childhood cancers are often associated with anemia, which is a low red blood cell count. Polycythemia is the opposite, or a high red blood cell count. Polycythemia is common in newborn infants, and usually goes away on its own. If it persists or is found beyond the newborn period it should be evaluated by a pediatric hematologist, although generally for conditions other than cancer.
Polycythemia. Polycythemia means having too many red blood cells. There are many causes for this and the treatment is directed at the underlying cause. You should see your primary doctor or a hematologist for an evaluation.