Medulloblastoma, Adult

The purpose of this research study is to test the safety and effectiveness of a cell therapy at different doses for children and young adults with recurrent or progressive brain tumors. Recurrent/recurred means a tumor that has gone away and then came back. This cell therapy is called B7- H3.CD28Z.CART, referred to as B7-H3 CAR T cells. B7-H3 is a protein that is over-expressed on many tumor cells, making it a good target for cancer cell therapy. The names of the study investigational therapies involved in this study are: * Fludarabine (a type of chemotherapy) * Cyclophosphamide (a type of chemotherapy) * B7-H3 CAR T cells (a type of cellular therapy)