Key Inclusion Criteria:
- Parts A & C: patients must be >= 12 months and < 18 years of age at the time of study enrollment
- Parts B1-B6, B8, D1-D6: patients must be >= 12 months and =< 30 years of age at the time of study enrollment
- Part B7: patients must be >= 12 months and < 18 years of age at the time of study enrollment
- Patients must have had histologic verification of malignancy at original diagnosis or relapseStandard COG Phase 1 organ function requirementsNo evidence of dyspnea at rest, no exercise intolerance due to pulmonary insufficiency, and a pulse oximetry > 92% while breathing room air
Key Exclusion Criteria:
- Patients with CNS tumors or known CNS metastases will be excluded from this trial; patients with a history of CNS metastases that have been previously treated may enroll if sequential imaging shows not evidence for active disease; patients with extra axial disease (e.g. skull [bone] metastasis that do not invade the dura) may enroll if there is no evidence for CNS edema associated with the lesionPatients with a history of any grade autoimmune disorder are not eligible; asymptomatic laboratory abnormalities (e.g. antinuclear antibody [ANA], rheumatoid factor, altered thyroid function studies) will not render a patient ineligible in the absence of a diagnosis of an autoimmune disorderPatients with >= grade 2 hypothyroidism due to history of autoimmunity are not eligible; note: hypothyroidism due to previous irradiation on thyroidectomy will not impact eligibilityPatients with a history of congestive heart failure (CHF) or are at risk because of underlying cardiovascular disease or exposure to cardiotoxic drugs must have adequate cardiac function as clinically indicated:Corrected QT interval (QTC) =< 480 msecShortening fraction of >= 27% by echocardiogram or ejection fraction of >= 50% by gated radionuclide studyPatients who have received prior solid organ transplantation are not eligiblePatients who have received prior anti-PD1 directed therapy (monoclonal antibody [mAb] or small molecule) are not eligible