This trial is for children and adults with high-risk neuroblastoma who have either not responded well to initial treatment (primary refractory disease) or who have relapsed after treatment (incomplete response to salvage treatment). The treatment involves naxitamab, which is a humanised monoclonal antibody targeting GD2, and granulocyte-macrophage colony stimulating factor (GM-CSF). Patients will receive treatment for up to 101 weeks and will be followed for up to five years after their first dose.