Covid-19; HSCT; MIS-C; Pediatric; alanine aminotransferase; amino terminal pro brain natriuretic peptide; aspartate aminotransferase; bilirubin; C reactive protein; creatinine; D dimer; ferritin; hemoglobin; immunoglobulin; interleukin 2; methylprednisolone; prednisolone; ruxolitinib; sirolimus; thymocyte antibody; artificial ventilation; blood pressure; breathing rate; case report; child; clinical article; coronavirus disease 2019; drug dose reduction; drug withdrawal; electrocardiography; female; heart rate; hematopoietic stem cell transplantation; human; immunosuppressive treatment; intensive care unit; Letter; leukocyte count; male; neutrophil count; oxygen saturation; pediatric multisystem inflammatory syndrome; plasma exchange; preschool child; rebreathing; responsibility; school child; thin layer chromatography; thorax radiography