 This study will assess the feasibility of utilizing a reduced intensity conditioning regimen, in the setting of haploidentical transplantation, for patients with recurrent acute lymphoblastic leukemia ALL, AML and high risk or refractory solid tumors. In addition, the feasibility and safety of administering post-transplant NK cell infusions will be evaluated. Data obtained from this study will help determine the efficacy of allogeneic HSCT in the treatment of pediatric sarcomas and add to the small body of literature utilizing haploidentical HSCT to treat acute leukemia in pediatric patients. This study will also further elucidate the role of NK cells in mediating a graft vs. tumor effect in allogeneic HSCT. The main benefit to society is that this study will explore a novel therapy for children with highly refractory cancer who are felt to be incurable with conventional approaches. If feasibility is demonstrated, and there is evidence of anti-tumor activity, then this will open up a new area of clinical research to better define the efficacy of this approach for specific childhood malignancies. 