Idelalisib With Rituximab, Ifosfamide, Carboplatin, Etoposide (RICE) in Children and Adolescents

Key Inclusion Criteria: * Histologically confirmed diagnosis of DLBCL or MBCL established by the World Health Organization (WHO) 2008 classification of tumors of hematopoietic and lymphoid tissues * Relapsed or refractory disease * Measurable or evaluable disease based on imaging or bone marrow examination * Karnofsky ≥ 60% for participants \> 16 years of age or Lansky ≥ 60 for participants ≤ 16 years of age. Participants who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing the performance score. * A negative serum pregnancy test is required for females of child bearing potential. * Participants of child bearing potential who engage in heterosexual intercourse must agree to use protocol specified method(s) of contraception . * Lactating females must agree to discontinue nursing before idelalisib is administered * Adequate bone marrow function as defined in the protocol * Adequate renal function as defined in the protocol Key Exclusion Criteria: * Prior ifosfamide, carboplatin, etoposide (ICE) therapy, with or without an anti-CD20 antibody, or history of hypersensitivity to any components of RICE * Known active central nervous system or leptomeningeal lymphoma or within 4 weeks from the last intrathecal therapy prior to the required diagnostic lumbar puncture (LP) for this study * Disease progression within 6 months from last anti-CD20 therapy * Ongoing toxicity from prior cytotoxic therapy (last dose at least 3 weeks prior to study entry) * Less than 4 half-lives from the last dose of previous targeted therapy and ongoing acute toxicity of prior targeted therapy * Active infection with human immunodeficiency virus (HIV), cytomegalovirus (CMV), hepatitis B virus (HBV), or hepatitis C virus (HCV) based on screening serology and polymerase chain reaction (PCR) results * Evidence of systemic bacterial, fungal, or viral infection at the time of treatment start (Day 1) * Ongoing or history of drug-induced pneumonitis * Ongoing or history of inflammatory bowel disease * Pregnancy or breastfeeding * Currently receiving other anti-cancer or other investigational drug * Prior solid organ transplantation * Prior allogeneic stem cell transplantation within 60 days or active acute graft versus host disease (GVHD) Grade 3 or higher * Known hypersensitivity to idelalisib, the metabolites, or formulation excipients * Any other clinical condition or prior therapy that, in the opinion of the Investigator, would make the participant unsuitable for the study or unable to comply with the study requirements NOTE: Other protocol defined Inclusion/ Exclusion criteria may apply.