This study enrolls patients that have been diagnosed with SMARCB1 or SMARCB4 deficient cancer that has either come back (“relapsed”) or does not respond to therapy (“is refractory”) or have been newly diagnosed with a SMARCB1 or SMARCB4 deficient cancer that has no known standard treatment. This study involves taking study drugs called tiragolumab and atezolizumab. These drugs are a type of immunotherapy that try to make the immune system fight cancer. The purpose of the study is to learn more about the safety of tiragolumab in children and to see how well the combination of tiragolumab and atezolizumab works when given to children and adults with these types of tumors. This study has two parts. In Part A, participants will receive tiragolumab. In Part B, participants will receive both tiragolumab and atezolizumab.