What is the purpose of this study? This phase II trial tests the safety and best dose of revumenib when given together with chemotherapy, and how well the treatment regimen works for infants and young children with leukemia that has come back (relapsed) or does not respond to treatment (refractory) and is associated with a KMT2A (MLL) gene rearrangement (KMT2A-R). Revumenib is an oral medicine that directly targets the changes that occur in a cell with a KMT2A rearrangement and has been shown to specifically kill these leukemia cells in test tubes and animals. Drugs used in chemotherapy, such as vincristine, prednisone, asparaginase, fludarabine and cytarabine work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. This trial is being done to find out if the combination of revumenib and chemotherapy may help to treat the cancer cells better than either treatment alone.THIS STUDY IS ENROLLING BY INVITATION ONLY - Consistent with most oncology trials, patients are not actively “recruited,” but are screened by their physician for appropriate clinical trial(s) at the time of their routine clinic visit. Occasionally, a patient may be a self-referral or physician referral, but are still screened for appropriate clinical trials at the time of their routine clinic visit. PI and staff may send copies of relevant consent forms to these patients to look over prior to actually consenting or enrolling them. This may take place at the patient's visit at which the consent is presented or the patient's next visit to the outpatient hematology/oncology clinic. Interested in participating? For more information about this research study or other cancer-related clinical trials at IU Simon Comprehensive Cancer Center, please contact:IU Clinical Trials Office Email: iutrials@iu.edu Phone: (317) 278-5632