Additional Study Information: The main purpose of the study is to find out about whether the study drug is safe in humans and aneffective treatment in pediatric patients with acute Graft versus Host Disease (GVHD). GVHD mayoccur after a bone marrow or stem cell transplant in which someone receives bone marrow tissue orcells from a donor (called an allogeneic transplant). The new, transplanted cells attack the recipient'sbody (the host) because they see it as foreign. The study drug, Remestemcel-L, is an investigational drug. Investigational means that the study drug is currently being tested and is not approved for sale in the United States by the Food and Drug Administration (FDA) or the European Medicines Agency (EMA). Remestemcel-L is made from adult human stem cells, derived from the bone marrow of healthy volunteers and cultured (grown) in a laboratory. Your child will receive Remestemcel-L, the study drug, as an intravenous (IV) infusion, which is a method to administer medication and/or fluids directly into a vein, twice per week for 4 weeks of initial therapy. The study drug will be administered at least 3 days apart. The dose will be determined based on your childs body weight taken at the first visit.