Additional Study Information: This is a phase 1/2 multicenter study of AZD1775 (MK-1775) that will be given in combination with irinotecan. The study treatment is considered experimental because it is not approved by the United States (US) Food and Drug Administration (FDA) for treating children and young adults with solid tumors which have come back or have not responded to standard therapy. The purpose of this study is to learn what kind of side effects MK-1775 in combination with irinotecan can cause and whether it is a beneficial treatment for solid tumors. Both drugs will be given by mouth for 5 days (from Day 1 to Day 5) of a 21-day cycle. Disease assessment will be completed before beginning treatment, before starting a new cycle, and at the end of study treatment. During cycle 1, blood samples will be collected to determine how much MK-1775 is in the blood (pharmacokinetics). If you agree to participate in optional studies, blood samples and bone marrow samples will be collected to determine how much MK-1775 is in the blood and bone marrow. Once the highest safe dose of MK-1775 in combination with irinotecan is determined, the study will open to patients with neuroblastoma, central nervous system (CNS) tumor, and rhabdomyosarcoma which have come back or have not responded to standard therapy (Part B, C, D of the study). Both drugs will be given by mouth for 5 days (from Day 1 to Day 5) of a 21-day cycle. Disease assessment will be completed before beginning treatment, before starting a new cycle, and at the end of study treatment. Cancer types: central nervous system tumors (CNS tumors), neuroblastoma, medulloblastoma or central nervous system PNET tumors (CNS PNET tumors), and rhabdomyosarcoma.