The Pediatric Acute Leukemia (PedAL) Screening Trial - A Study to Test Bone Marrow and Blood in Children With Leukemia That Has Come Back After Treatment or Is Difficult to Treat - A Leukemia & Lymphoma Society and Children's Oncology Group Study

Inclusion Criteria: * Patients must be less than 22 years of age at the time of study enrollment * Patient must have one of the following at the time of study enrollment: * Patient has known or suspected relapsed/refractory (including primary refractory) AML as defined in protocol * This includes isolated myeloid sarcoma * Patient has known or suspected relapsed/refractory (including primary refractory) myeloid leukemia of Down syndrome (ML-DS) * Patient has known or suspected relapsed ALL as defined in protocol that meets one of the following criteria: * Second or greater B-ALL medullary relapse, excluding KMT2Ar * Any first or greater B-ALL medullary relapse involving KMT2Ar * Any first or greater T-ALL medullary relapse with or without KMT2Ar * Patient has known or suspected relapsed/refractory (including primary refractory) mixed phenotype acute leukemia (MPAL) as defined in protocol * Patient has known or suspected de novo or relapsed/refractory (including primary refractory) treatment-related AML (t-AML) * Patient has known or suspected de novo or relapsed/refractory (including primary refractory) myelodysplastic syndrome (MDS) or treatment-related myelodysplastic syndrome (t-MDS) * Note: Relapsed/refractory disease includes stable disease, progressive disease, and disease relapse. * Patient has known or suspected de novo or relapsed/refractory (including primary refractory) juvenile myelomonocytic leukemia (JMML) * Note: Relapsed/refractory disease includes stable disease, progressive disease, and disease relapse. * All patients and/or their parents or legal guardians must sign a written informed consent * All institutional, Food and Drug Administration (FDA), and National Cancer Institute (NCI) requirements for human studies must be met