This study enrolls patients that have been diagnosed with acute leukemia associated with a KMT2A (MLL) gene rearrangement (referred to as KMT2Arearranged, or KMT2A-R). The KMT2A-R leukemia has come back after treatment (relapsed) or is not responding to treatment (it is refractory). This study involves taking a study drug called SNDX-5613. The study is divided into two phases: one in which the study drug is given in combination with chemotherapy and a phase where the study drug is given alone. The purpose of the study is to test the safety of the study drug with chemotherapy, and how well the treatment regimen works for infants and young children with relapsed or refractory KMT2A-R leukemia.