Additional Study Information: This study is for children with relapsed or refractory acute myeloid leukemia (rrAML). Acute myeloid leukemia is a fast growing cancer of the blood and bone marrow. This study examines the effects of study drug Lenalidomide on children with rrAML. Lenalidomide is an anti-cancer drug that belongs to a group of drugs known as immunomodulating drugs (IMiDs), a medicine that can modify or regulate the functioning of the immune system. These drugs change how diseased cells in the blood and bone marrow grow. This study also includes testing of bone marrow samples for biomarkers. Biomarkers aresubstances such as gene or protein changes that tell us how the drug is working in your childsbody.