Additional Study Information: The purpose of this study is to determine if a study drug called pembrolizumab is safe and effective in the treatment of children and adolescents who have advanced melanoma, or advanced, relapsed, or refractory PD-L1 positive malignant solid tumor or other lymphoma, relapsed or refractory classical Hodgkin lymphoma (rrcHL), advanced, relapsed or refractory microsatellite-instability-high (MSI-H) solid tumor. Participants might be at least 6 months ( 3 years rrcHL) to less than 18 years old and able to provide the sample for testing the PD-L1 (programmed cell death ligand 1) purpose from an archival tissue sample or newly obtained biopsy to determine if might qualify for this research study. The participant may be in the study for up to approximately 52 cycles, each study therapy cycle is every 3 weeks.