A Study to Find the Highest Dose of Imetelstat in Combination With Fludarabine and Cytarabine for Patients With AML, MDS or JMML That Has Come Back or Does Not Respond to Therapy

Inclusion Criteria: * Patients must be ≥ 1 year and ≤ 18 years of age at the time of study enrollment * Patients, with or without down syndrome (DS), and with acute myeloid leukemia, therapy-related AML, MDS or JMML and meet one of the following: * Second or greater relapse or refractory AML, including isolated extramedullary disease (EMD), but excluding isolated central nervous system (CNS) or isolated testicular relapse * First or greater relapse of MDS * First or greater relapse of JMML * For flow cytometry, it's strongly recommended to enroll onto APAL2020SC or to send samples to Hematologics, Inc. Otherwise, assessments must be performed at a College of American Pathologists (CAP)/Clinical Laboratory Improvement Act (CLIA) certified lab that has expertise in AML. * For fluorescence in situ hybridization (FISH)/Karyotype, samples must be sent to a Children's Oncology Group (COG)-approved Cytogenetics Lab * Bone marrow relapse AML: (patients must meet one of the following criteria to be defined as having relapsed disease) * A single bone marrow sample showing ≥ 5% leukemic blasts by flow cytometry, fluorescence in situ hybridization (FISH) testing, or other molecular method * A single bone marrow with at least two tests showing ≥ 1% leukemic blasts; examples of tests include: * Flow cytometry showing ≥ 1% leukemic blasts by multidimensional flow cytometry (MDF) * Karyotypic abnormality with at least one metaphase similar or identical to diagnosis …