Vitamin D acts on cells of the immune system and causes many effects, including the production of a "natural antibiotic" called cathelicidin. The investigators know that when people are supplemented with vitamin D, levels of cathelicidin produced by these immune cells increase. By supplementing children with Crohn's disease with vitamin D, the investigators may be able to alter their immune system "naturally," making their disease better. A consensus of vitamin D experts believes that vitamin D levels need to reach a level of 40-70 ng/mL in the blood in order to have effects on the immune system. Raising vitamin D levels to this range is one of the goals in the current study.