The primary objective of this study is to evaluate the toxicity (as measured as 100 day survival) after hematopoietic stem cell transplant from an unrelated donor with a novel preparative regimen of Fludarabine, Busulfan, Anti-Thymocyte Globulin, and Total Body Irradiation for pediatric patients with leukemia. The secondary objectives are to evaluate the relapse-free and overall survival after hematopoietic stem cell transplant as well as to evaluate the incidence of acute and chronic graft-versus-host disease after this preparative regimen.