Additional Study Information: The purpose of this study is to find out more about the effectiveness (how well the drug works), safety, and tolerability of an investigational drug called nivolumab (also known as BMS-936558) when given alone or when combined with ipilimumab in children/ adolescents/ young adults with high grade primary central nervous system (CNS) malignancies (certain types of brain tumor). This is a phase 1b/ 2 study, multicenter, open-label, that means all subject enrolled in the study will be assigned to nivolumab monotherapy (Module A) and/or nivolumab plus ipilimumab (Module B). Male or female subjects aged 6 months to 22 years will be enrolled, participant must have received standard of care. The study will consist of four parts: Screening, Treatment, Safety Follow-Up, and Long-Term Follow-up. Cancer types include: diffuse intrinsic pontine glioma (DIPG), high grade glioma (HGG), medulloblastoma, ependymoma, or other high-grade tumors of the central nervous system.