This study will be testing an experimental immunotherapy drug called indoximod which may help the body attack tumor cells more effectively when combined with standard chemotherapy and radiation therapy. The study will enroll patients 3 to 21 years of age diagnosed with brain cancer (glioblastoma, medulloblastoma, or ependymoma) or diffuse intrinsic pontine glioma (DIPG). During the study, indoximod will be taken by mouth twice a day while receiving standard cancer treatment. Because indoximod is an experimental drug, side effects will be monitored during the study. Patients in the study will return for clinic (cycle) visits every 28 days. It is hoped that combining indoximod with standard chemotherapy and radiation therapy treatments will improve patient outcomes with these types of cancers.