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 not yet approved for use in children or adults. 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.