This study enrolls patients that have been diagnosed with acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), or mixed phenotype acute leukemia (MPAL) that has either come back (‘relapsed’) or does not respond to therapy (‘refractory’). This study involves taking a drug called uproleselan that is approved for adults with AML but is not yet approved for children. The purpose of the study is to find a safe dose of the study drug in children that can be given with standard chemotherapy without causing severe side effects.