This research is being done to study the effectiveness of vitamin D (cholecalciferol) to modify hepcidin levels in children with chronic kidney disease (CKD). Anemia is a common problem in children with CKD. Anemia is when the body does not have enough healthy red blood cells. Hepcidin is a protein in the blood which interferes with the body's production of red blood cells. This study will see if vitamin D lowers hepcidin levels in children and young adults with CKD. If so, it could be used as an additional treatment for anemia in these children, in addition to the current therapies already in use including iron supplements and erythropoietin. People between the ages of 1 and 21 with CKD may be considered for this study.