Additional Study Information: This is a Phase 1 study of a drug called uproleselan. We are testing new experimental drugs such as uproleselan in the hopes of finding a treatment that may be effective against acute myeloid leukemia, myelodysplastic syndrome or mixed phenotype acute leukemia that has come back or that has not responded to standard therapy. This study looks at how well uproleselan works when given to children and young adults with relapsed or refractory leukemia. Uproleselan is experimental because it has not been proven to work in this type of situation. We are using uproleselan because it seems to work against cancer in test tubes and animals. Uproleselan has been used in adults and there is a lot that we do not know about it yet.