Additional Study Information: The purpose of this study is to find out if the drugs called Nivolumab and 5-Azacytidine can be given safely in children and young adults with Acute Myeloid Leukemia (AML) which has come back after treatment or has not responded to standard therapy. These drugs are thought to work by turning on genes that limit the growth of cancer cells. The study drugs will be given for 2 courses of chemotherapy duration based on how well the participants will tolerate the study drug and response, each course is 29-36 days long. Treatment for the 2 courses of therapy will last about 2-3 months.