The purpose of this study is to evaluate the safety and efficacy of PROMOSTEM (human umbilical cord blood-derived mesenchymal stem cells) at a dose of 1 and 5x1,000,000 hMSC/kg in subject for the promotion of an engraftment and prevention of graft rejection and Graft-Versus-Host Disease after unrelated hematopoietic stem cell transplantation for children with acute leukemia.