Although a majority of children with leukemia and most hematological malignancies (Hodgkin's and Non-Hodgkin's lymphomas) can be cured with conventional chemotherapy, a subset of patients with resistant/recurrent high-risk disease are not cured with conventional treatment regimens. Investigators hypothesize that HSCT from a partially matched donor can be safe and effective for patients with very high risk hematologic malignancies when combined with post-transplant cyclophosphamide for prevention of graft-vs-host disease (GVHD).