PEP-CMV + Nivolumab for Newly Diagnosed Diffuse Midline Glioma/High-grade Glioma and Recurrent Diffuse Midline Glioma/High-grade Glioma, Medulloblastoma, and Ependymoma

Inclusion Criteria for All Patients: * Patients must be ≥4 and ≤25 years of age (inclusive) at the time of study enrollment * Metastatic Disease: Patients with M+ disease are eligible. * Adequate bone marrow function defined as: * ANC (Absolute neutrophil count) ≥ 1000/µl. * Platelets ≥ 75,000/µl. * Hemoglobin \> 8 g/dL. (may be supported) * Adequate renal function defined as: * Creatinine clearance or radioisotope GFR ≥ 70ml/min/1.73 m\^2 OR A serum creatinine based on age/gender as listed in the protocol. Note: The threshold creatinine values in this table were derived from the Schwartz formula for estimating GFR utilizing child length and stature data published by the CDC. * Adequate liver function defined as: * Total bilirubin ≤1.5 times institutional ULN * AST(SGOT) ≤3 × institutional upper limit of normal * ALT(SGPT) ≤3 × institutional upper limit of normal * The effects of PEP-CMV and nivolumab on the developing human fetus are unknown. For this reason, female participants of childbearing potential and male participants who are sexually active must agree to use adequate contraception prior to study entry, for the duration of study participation, and for at least 6 months after completion of study participation. Pregnancy tests must be obtained in girls and women who are post-menarchal at screening, at least 24 hours prior to the first dose of nivolumab and every 4 weeks (+/- 1 week) regardless of dosing schedule. An extension up to 72 hours prior to t…