Posterior Fossa-A Ependymoma

This study is researching an experimental drug called cemiplimab (called "study drug") and the combination of experimental drugs of fianlimab and cemiplimab (called "study drugs"). The study is focused on children and young adults with recurrent or progressive High-Grade Glioma (HGG) or ependymoma. "Recurrent" means that the cancer came back after treatment. "Progressive" means that the tumor has grown or spread. The aim of the study is to see how safe, tolerable, and effective cemiplimab and the combination of fianlimab and cemiplimab are. The study is looking at several other research questions, including: * What side effects may happen from receiving the study drug(s) * Do the study drug(s) help study participants live longer without their tumors growing or spreading * How much of the study drug(s) is in the blood at different times * Whether the body makes antibodies against the study drug(s) (which could make the study drug\[s\] less effective or lead to side effects)