Nivolumab in Combination With Metronomic Chemotherapy in Paediatrics Refractory / Relapsing Solid Tumors

INCLUSION CRITERIA: * Histologically proven diagnosis of solid malignant tumor. Confirmed progressive or refractory disease despite standard therapy or for which no effective standard therapy exists * Histologically proven diagnosis of: embryonal brain tumor ; ependymoma ; low-grade glioma (LGG) ; high-grade glioma (HGG) except diffuse Intrinsic Pontine glioma (DIPG) Supratentorial Diffuse Midline Glioma K27M mutated are eligible ; rhabdomyosarcoma ; neuroblastoma ; Ewing sarcoma ; and other solid tumors and after approval from coordinators (except DIPG, osteosarcoma, lymphoma), and confirmed progressive or refractory disease despite standard therapy or for which no effective standard therapy exists (this criterion is applicable to stage 2 only) * Male and female subjects \< 18 years of age at inclusion; patients of 18 years and older may be included after discussion with the sponsor if they had a pediatric recurrent/refractory malignancy diagnosed before the age of 18. * Evaluable or measurable disease as defined by adequate standard imaging criteria for each patient's tumor type (see corresponding appendices for definition of evaluable and/or measurable lesions): * RANO criteria for patients with high grade glioma (HGG), who are eligible at stage 1 only * RAPNO criteria for patients with low grade glioma * WHO for other cerebral tumors * INRC criteria for patients with neuroblastoma (NB), * RECIST v1.1 for tumors other than cerebral tumors and neuroblastoma * Pe…