Stem cell transplant is an important therapeutic option for pediatric patients with relapsed or refractory leukemia. Although, full myeloablative transplants are widely used for patients with acute leukemia, myeloablative chemo-radiotherapy may not be feasible in some specific settings. These settings include 1) patients with pre-existing health issues and organ toxicities; 2) patients who have relapsed post-ablative transplant and need a second stem cell transplant; and 3) leukemia patients with advanced disease who have been heavily pre-treated. Clofarabine, a new purine nucleoside anti-metabolite, has the advantage of significant antileukemic activity in addition to its possible immuno-suppressive properties. In this study we plan to determine the maximum feasible dose (MFD) of Clofarabine in combination with total body irradiation that can achieve durable donor engraftment without causing excessive toxicity.