A Pilot Study to Evaluate the Feasibility of Post-Hematopoietic Stem Cell Transplant Prophylaxis With Decitabine Combined With Filgrastim for Children and Young Adults With AML, MDS and Related Myeloid Malignancies

The purpose of this study is to examine if it is feasible to administer decitabine and filgrastim after allogenic hematopoietic stem cell transplant (HCT) in children and young adults with myelodysplastic syndrome, acute myeloid leukemia and related myeloid disorders, and if the treatment is effective in preventing relapse after HCT. The names of the study drugs involved in this study are: * Decitabine (a nucleoside metabolic inhibitor) * Filgrastim (a recombinant granulocyte colony-stimulating factor (G-CSF)

Inclusion Criteria: * Disease Criteria: Participants must have a histologically confirmed diagnosis of one of the following hematologic malignancies for eligibility, as defined by the criteria below: * AML (relapsed, de-novo or secondary) based on WHO classification * MDS (relapsed, de-novo or secondary) based on WHO classification * Treatment myeloid neoplasm (tMDS/AML; relapsed disease included) * Myeloid Sarcoma * Acute Undifferentiated Leukemia (MPAL and acute leukemia of ambiguous lineage/NOS not eligible) * Note: MDS, AML, MDS/AML, or tMDS/AML as defined above may be idiopathic/de novo or derived from a germline predisposition to myeloid malignancy. For patients with an underlying germline disorder, those conditions that are not associated with increased risk for toxicity to treatment, including patients with known germline ANKRD26, DDX41, ELANE and other congenital neutropenia disorders, ETV6, GATA-2, Li-Fraumeni, RUNX1, SAMD9/SAMD9L, or Shwachman-Diamond Syndrome, will be analyzed within the general treatment cohort (Cohort A, see Table 1) along with patients with idiopathic disease (Cohort B, see Table 2). MDS, AML, MDS/AML, or tMDS/AML derived from the following germline disorders will be enrolled in a separate cohort (B) and adverse events monitored closely for higher rates compared to cohort A: * Dyskeratosis Congenita or associated telomeropathies as defined by telomere length \ Grade 2) except for bone marrow suppression. * Participants should not be enrolled on another study that prohibits initiation of maintenance therapy. * History of allergic reactions attributed to compounds of similar chemical or biologic composition to decitabine or filgrastim. * Participants with uncontrolled intercurrent illness. * Participant who are not able to present for clinic visits for at least 7 months after study treatment initiation. * Participant with FLT3/ITD mutations are excluded as maintenance therapy with tyrosine kinase therapy should be considered in this context. However, if a participant has a co-occurring NUP28 mutation, they will be considered eligible. * Participants with a concurrent active malignancy are not eligible for this trial.