 The primary hypothesis of this study is that intravenous IV administration of pembrolizumab to children with either advanced melanoma; a PD-L1 positive advanced, relapsed or refractory solid tumor or other lymphoma; advanced, relapsed or refractory MSI-H solid tumor; or rrcHL, will result in an Objective Response Rate ORR greater than 10 for at least one of these types of cancer. 