The investigators will conduct a phase 1 trial of Selinexor in children with recurrent or refractory solid tumors, including CNS tumors using the Rolling Six design. The aims of the trial will be to establish the maximum tolerated pediatric doses of tablet formulation; to investigate the toxicities, pharmacokinetics, and pharmacodynamics of Selinexor in children with cancer; and to preliminarily explore efficacy in pediatric solid and CNS tumors, including medical and surgical expansion cohorts of HGG patients.