Additional Study Information: This study is being done to see if giving your child a lower dose of chemotherapy (drug therapy), than has been used historically in bone marrow transplant, to kill his/her own diseased cells before he/she receives a bone marrow transplant, will possibly result in the permanent control of your child's sickle cell disease. Patients will receive busulfan, fludarabine and alemtuzumab prior to the stem cell transplant to help the stem cells take and grow. The stem cells will be infused on day 0 through an intravenous catheter. Patients will receive immunosuppressive therapy such as tacrolimus and MMF to prevent graft-versus-disease. Patients will be followed up after transplant to look for special cells in the blood that show that the new bone marrow is taking hold (engrafting). Response shall be documented at 6 months, 1 year, 2 years and 3 years post-SCT.