This study is for young patients with certain brain tumors like gliomas (fast-growing brain tumors), diffuse intrinsic pontine gliomas (brain stem tumors), and others that haven't responded to usual treatments. The study uses a drug called pembrolizumab, a type of immunotherapy. Immunotherapy helps the body's immune system to fight cancer by using special proteins called monoclonal antibodies. In this study, doctors want to find the best dose of pembrolizumab and see its side effects.

Patients will receive pembrolizumab through an IV every 21 days for up to 34 cycles. They will also have MRI scans and give blood samples to check their progress. After treatment, follow-ups will be done for up to 3 years. Participants must have had previous treatments and be between 1 to 18 years old. They need to be stable enough to join the study.

• This study lasts for up to 3 years with follow-ups.
• Participants will receive treatment every 21 days in cycles.
• Must have had previous cancer treatments and meet specific health criteria.