* Patients must be 1 month to \< 6 years old at the time of study enrollment and must have had initial diagnosis of leukemia at \< 2 years old. * Patients must have KMT2A-rearranged acute lymphoblastic leukemia (ALL), acute leukemia of ambiguous lineage (ALAL), or mixed phenotype acute leukemia (MPAL), which is determined to be refractory or in first marrow relapse. All patients must undergo cytogenetics and fluorescence in situ hybridization (FISH) testing of a relapsed/refractory blast sample at a Children's Oncology Group (COG)-approved laboratory for KMT2A-R status determination and the presence of a KMT2A- rearrangement must be confirmed by central review. Cytogenetics results must be submitted for central review by Day 10 of protocol therapy, for confirmation of KMT2A-R status. Patients enrolled with refractory disease may utilize initial diagnostic cytogenetics for eligibility and submission for central review if testing was performed at a COG approved laboratory. Patients will be eligible to remain on protocol therapy if KMT2A-R is confirmed by central review. Additional methods of assessing for KMT2A-R may be considered if FISH does not detect the rearrangement. * Disease status at time of enrollment must be one of the following: * First relapse (untreated): Any recurrence of marrow disease, with or without other extramedullary sites(s), at any point after achieving remission ("remission-1", per definition below) and meeting one of the below criteria. Patients must not have received any disease-directed therapy for the marrow relapse prior to enrollment, other than permitted cytoreduction. * Relapse M1: M1 morphology (\< 5% blasts) + at least 2 confirmatory tests showing \>= 1% blasts (testing includes flow, cytogenetics, polymerase chain reaction \[PCR\]/next-generation sequencing \[NGS\] of immunoglobulin \[Ig\]/T-cell receptor \[TCR\] rearrangement, and/or PCR or NGS of fusion gene identical to diagnosis), OR * Relapse M2: M2 morphology (5-25% blasts) + 1 confirmatory test showing \> 1% blasts, OR * Relapse M3: M3 morphology (\> 25% blasts) * Primary refractory, or failure to achieve remission-1: remission-1 is defined as \< 1% marrow blasts by flow MRD and resolution of extramedullary disease following at least 2 courses of frontline chemotherapy. Patients who receive 2 courses of chemotherapy and 1 course of blinatumomab are also eligible, but no further treatment attempts beyond that are permitted * Central nervous system (CNS) disease: Patients must have CNS1 or CNS2 status and no clinical signs or neurologic symptoms suggestive of CNS leukemia, such as cranial palsy. * Patients with CNS3 disease may receive antecedent intrathecal chemotherapy to achieve CNS1 or CNS2 status prior to enrollment. * Patients with a history of CNS chloromatous disease are required to have no radiographic evidence of CNS disease prior to enrollment. * White blood cell (WBC) must be \< 50,000/uL at the time of study enrollment. Patients can receive cytoreduction with hydroxyurea and/or corticosteroids for up to 7 days prior to enrollment. * Patients \>= 12 months of age must have a performance status by Lansky Scale of \>= 50%. * Patients must be able to take enteral medications. Acceptable routes of administration for revumenib (SNDX-5613) include: oral (PO), nasogastric (NG) tube, nasojejunal (NJ) tube, nasoduodenal (ND), and gastrostomy tube (G-tube). * Patients must have fully recovered from the acute toxic effects of all prior chemotherapy, immunotherapy, or radiotherapy prior to entering this study * Cytotoxic chemotherapy or other anti-cancer agents known to be myelosuppressive: * \>= 14 days must have elapsed after the completion of other cytotoxic therapy, including patients who relapse during pre-Maintenance upfront therapy, with these specific exceptions: cytoreduction with hydroxyurea and/or corticosteroids, and intrathecal chemotherapy, which have no required washout periods. For patients who relapse during upfront Maintenance therapy, \>= 7 days must have elapsed after the last dose of chemotherapy. Additionally, patients must have fully recovered from all acute toxic effects of prior therapy. * NOTE: Cytoreduction with hydroxyurea and/or corticosteroids is permitted prior to enrollment for patients with WBC \>= 50,000/uL, and by provider discretion regardless of WBC, to reduce potential risk of differentiation syndrome with revumenib initiation. Hydroxyurea and/or corticosteroids may be given for up to 7 days, with no wash-out required. * NOTE: No waiting period is required for patients having received intrathecal cytarabine, methotrexate, and/or hydrocortisone. Intrathecal chemotherapy that is given up to 7 days prior to the initiation of protocol therapy counts as protocol therapy and not prior anti-cancer therapy. Intrathecal chemotherapy given \> 7 days prior does not count as protocol therapy. * NOTE: Prior exposure to fludarabine and cytarabine (FLA) is permitted. * Anti-cancer agents not known to be myelosuppressive (e.g., not associated with reduced platelet or absolute neutrophil count \[ANC\] counts): \>= 7 days after the last dose of agent. * Antibodies: \>= 21 days must have elapsed from infusion of last dose of antibody, and toxicity related to prior antibody therapy must be recovered to grade =\< 1. There is an exception for blinatumomab infusions, for which patients must have been off for at least 3 days and all drug related toxicity must have resolved to grade 2 or lower as outlined in the inclusion/exclusion criteria. * Hematopoietic growth factors: \>= 14 days after the last dose of a long-acting growth factor (e.g., pegfilgrastim) or \>= 7 days for short-acting growth factor. For agents that have known adverse events occurring beyond 7 days after administration, this period must be extended beyond the time during which adverse events are known to occur. The duration of this interval must be discussed with the study chair and the study-assigned Research Coordinator. * Interleukins, interferons and cytokines (other than hematopoietic growth factors): \>= 21 days after the completion of interleukins, interferon, or cytokines * Stem cell infusions (with or without total body irradiation (TBI): * Allogeneic (non-autologous) bone marrow or stem cell transplant, or stem cell boost: \>= 84 days after infusion * Donor leukocyte infusion: \>= 28 days * Cellular therapy: \>= 28 days after the completion of any type of cellular therapy (e.g., modified T cells, natural killer \[NK\] cells, dendritic cells, etc.) * Radiation therapy (XRT)/external beam irradiation including protons: \>= 14 days after local XRT; \>= 84 days after TBI, craniospinal XRT or if radiation to \>= 50% of the pelvis; \>= 42 days if other substantial bone marrow radiation. * A creatinine based on age as follows: * Age 1 month to \< 6 months: maximum creatinine 0.4 mg/dL * Age 6 months to \< 1 year: maximum creatinine 0.5 mg/dL * Age 1 to \< 2 years: maximum creatinine 0.6 mg/dL * Age 2 to \< 6 years: maximum creatinine 0.8 mg/dL OR * a 24-hour urine creatinine clearance \>= 70 mL/min/1.73 m\^2 OR * a glomerular filtration rate (GFR) \>= 70 mL/min/1.73 m\^2. GFR must be performed using direct measurement with a nuclear blood sampling method OR direct small molecule clearance method (iothalamate or other molecule per institutional standard). * NOTE: Estimated GFR (eGFR) from creatinine, cystatin C or other estimates are not acceptable for determining eligibility. * A direct bilirubin =\< 1.5 x upper limit of normal (ULN) for age, unless disease related * Serum glutamic-pyruvic transaminase (SGPT) (alanine aminotransferase \[ALT\]) =\< 135 U/L (3 x ULN) unless disease related. * Note: For the purpose of eligibility, the ULN for SGPT (ALT) has been set to the value of 45 U/L * Shortening fraction of \>= 27% by echocardiogram, or ejection fraction of \>= 50% by radionuclide angiogram. * Corrected QT interval using Fridericia formula (QTcF) of \< 450 msec (using the average of triplicate measurements) * NOTE: There are no specific electrolyte parameters for eligibility. However, it should be noted that, to limit QTc prolongation risk, patients must maintain adequate potassium and magnesium levels to initiate and continue revumenib (SNDX-5613) on protocol therapy. * Patients must be able to comply with the safety monitoring requirements of the study, in the opinion of the treating investigator.