Microsatellite-instability-high Solid Tumor

Researchers are looking for new ways to treat children with different types of melanoma (skin cancer), solid tumors, and lymphomas (blood cancers) that are any of these: * Advanced, which means cancer spread in the body or cannot be removed with surgery * Relapsed, which means cancer has come back after it had responded to previous treatment (responded means it stopped growing, gets smaller, or disappeared) * Refractory, which means cancer did not respond to previous treatment Pembrolizumab is an immunotherapy, which is a treatment that helps the immune system fight cancer. Researchers want to learn if different doses of pembrolizumab can cause at least 1 of the types of cancer to get smaller or go away. With Amendment 8, enrollment of participants with solid tumors and participants 6 months to under 12 years old with melanoma were closed. With Amendment 13, enrollment was closed for participants with relapsed refractory classical Hodgkin lymphoma (rrCHL), microsatellite instabilty-high (MSI-H) solid tumors, tumor-mutational burden-high (TMB-H) solid tumors, and participants 12 years old to \<18 years old with advanced melanoma.