A Study to Find the Highest Dose of SNDX-5613 (Revumenib) as a Treatment Option After Hematopoietic Stem Cell Transplant in Children With Acute Lymphoblastic Leukemia, Acute Myeloid Leukemia, and Mixed Phenotype Acute Leukemia

Inclusion Criteria: * STEP 0 PRE-TRANSPLANT INCLUSION CRITERIA: * Patients must be ≥ 30 days and \< 22 years of age * PLEASE NOTE: Eligibility criteria to enroll onto Step 1 for the treatment trial is \< 22 years of age at the time of Step 1 enrollment. Please plan accordingly to ensure that patients who are screened with Step 0 will be at an eligible age at the time of enrollment onto Step 1. Patients who are 21 at the time of screening who turn 22 at the time of enrollment onto Step 1 will not be eligible to enroll onto the study * Patients with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), or mixed phenotype acute leukemia (MPAL) with a KMT2a rearrangement, NUP98 rearrangement, or NMP1 mutation confirmed in a College of American Pathologists (CAP)/Clinical Laboratory Improvement Act (CLIA) certified laboratory. Patients with a history of isolated or combined central nervous system (CNS) or extramedullary disease are eligible if they have no evidence of active CNS or extramedullary disease at the time of trial enrollment (Step 0) and treatment enrollment (Step 1). Eligible patients with histories of isolated or combined CNS or extramedullary disease at time of relapse are required to be in complete remission at time of transplant to be eligible for this study * AML and MPAL must be in morphologic complete remission confirmed by multiparameter flow (MDF) testing (with or without detectable minimal residual disease \[MRD\]). ALL must have bone marrow M…