A Study of the Drug IMGN632 in Children With Leukemia That Has Come Back After Treatment or is Difficult to Treat

Inclusion Criteria: * Patients from Children's Oncology Group (COG) sites must be enrolled on APAL2020SC * Patients must be \>= 12 months and less than 22 years of age at the time of study enrollment * Patient's weight at time of enrollment must be \>= 10 kg * Patients must meet the eligibility in the appropriate Cohort for their diagnosis and disease status * Cohort 1 (Phase 1 Monotherapy): * Patients must meet all the following criteria: * Patient must have bone marrow sample showing \>= 5% leukemic blasts by flow cytometry * Patient must have been diagnosed with AML, B-ALL, T-ALL, or mixed phenotype acute leukemia (MPAL) meeting one of the following disease criteria: * Second or greater relapse OR * Disease that is refractory to relapse therapy * Refractory to relapse therapy is defined as persistent disease after at least one cycle of induction chemotherapy to treat the relapse disease * Patient must have leukemic blasts that are CD123-positive by flow cytometry as determined either by the treating institution or through reference laboratory testing. * Cohort 2 (Combination Dose Finding) and Cohort 3 (Randomized Phase 2 Combination): * Patients must meet all the following criteria: * Patient must have AML in first relapse with a bone marrow sample showing \>= 1% leukemic blasts by flow cytometry * Patient must have leukemic blasts that are CD123-positive by flow cytometry as determined either by t…