Additional Study Information: This study will enroll with children, adolescents and young adults that are less than 22 years old that have returned or 'relapsed' acute myeloid leukemia (AML) diagnoses. Within this group, the study is also targeting those are also unable to receive additional chemotherapy treatments containing anthracycline, which is a type of antibiotic that is used to treat many types of cancer and can cause damage to the heart in some children and young adults. The study doctors want to find out how safe the new study drug called venetoclax is when given with three other chemotherapy drugs called fludarabine, cytarabine and gemtuzumab ozogamicin (FLA-GO). In this study, half of the participants will be given the study drug, venetoclax together with other chemotherapy drugs (FLA-GO) and half of the participants will only receive the other chemotherapy drugs (FLA-GO). Participants benefiting from treatment and who are not able to proceed to HSCT have the possibility to continue to receive azacitidine in monotherapy (Arm A, control arm) or in combination with venetoclax (Arm B, experimental arm). Participants can receive up to two cycles of induction chemotherapy before hematopoietic stem cell transplantation (HSCT). Participants benefiting from treatment and who are not able to proceed to HSCT have the possibility to continue to receive azacitidine in monotherapy (Arm A, control arm) or in combination with venetoclax (Arm B, experimental arm). Other procedures or tests will include physical examination, blood tests, electrocardiogram (ECG; to see how the heart is beating), X-ray, computed tomography (CT) scans and/or magnetic resonance (MRI) scan (to take pictures of the inside of the body) and blood tests. The study doctor plans to collect extra blood to measure the amount of venetoclax in participants' blood. Participants will receive the study treatment for up to 2 cycles and this will last about 10-12 weeks. They will be followed for up to 5 years after completing the study.