Additional Study Information: This study is being carried out by the Childrens Oncology Group (COG) Phase 1 Consortium. COG is an international research group that consists of more than 200 hospitals that treat children with cancer in the United States, Canada, Australia, and Switzerland. The Phase 1 Consortium is the group within COG that consists of 21 hospitals, and participation in this study will be limited to these hospitals. This is a Phase 1 study of a drug called entinostat. Entinostat is considered experimental because it has not been proven to work in a situation like yours. We are using entinostat because it seems to work against cancer in test tubes and animals. Entinostat has been used in adults, but there is a lot that we do not know about it yet. This is called a Phase 1 study because the goal is to find the highest dose of entinostat that we can give safely. We are testing new experimental drugs such as entinostat in the hopes of finding a treatment that may be effective against tumors or lymphoma that have come back or that have not responded to standard therapy.