Venetoclax Combined With Vyxeos (CPX-351) for Participants With Relapsed or Refractory Acute Leukemia

Inclusion Criteria: * Ages 1 Year to 39 Years * Diagnosis of one of the following: * Acute myeloid leukemia (AML), any subtype except * Patients with acute promyelocytic leukemia (APML) are NOT eligible * Patients with ML-DS are NOT eligible * Myeloid sarcoma * Acute leukemia of ambiguous lineage (ALAL) * Acute undifferentiated leukemia (AUL) * T/myeloid mixed phenotype acute leukemia (MPAL) * B/myeloid MPAL * MPAL with KMT2A-rearrangement MPAL with t (9;22) are NOT eligible * T-cell acute lymphoblastic leukemia (T ALL) * Early thymocyte precursor (ETP) ALL * KMT2A-rearranged ALL * Disease Status * Relapsed/Refractory AML, MPA, and AUL * Untreated therapy related AML * Relapsed/Refractory KMT2A-rearranged ALL, T-cell ALL, ETEP ALL * Karnofsky/Lanksy performance level score of greater than or equal to 50 percent. * Prior therapy requirements * Fully recovered from acute toxicities of Hematopoietic Stem Cell Transplant (HSCT) or Anthracycline Exposure * 14 days must have elapsed since the completion of systemic cytotoxic therapy other than hydroxyurea, decitabine or azacitidine * 2 weeks must have elapsed for local palliative radiotherapy (RT); 6 months must have elapsed if prior craniospinal RT or if 50% radiation of pelvis, and at least 6 weeks must have elapsed if other substantial bone marrow radiation * Adequate renal, liver, cardiac, and central nervous system (CNS) function Exclusion Criteria: * Diagnosis of one of the following: * Myeloid Leukemia associated with Down Syndrome (ML-DS) * Acute Promyelocytic Leukemia (APML) * Acute leukemia with CNS status 3 involvement * Philadelphia chromosome t(9;22) positive leukemia (Ph+ ALL, AML, MPAL, or AUL) * Fanconi Anemia, Shwachman-Diamond syndrome, or any other bone marrow failure syndrome or DNA repair disorder * Wilson's Disease or other copper-metabolism disorder * Pregnant or breastfeeding * Uncontrolled infection * Received greater than 13.6 Gray (Gy) prior radiation to the mediastinum * Receipt of growth factors within 7 days prior to enrollment * Currently receiving another investigational drug * Currently receiving anti-cancer agents (with the exception of intrathecal (IT) agents or hydroxyurea) * Unable to comply with the safety monitoring requirements of the study