The purpose of this research study is to determine if certain markers in your blood are increased over the duration of your chemotherapy, especially as you get sicker, such as developing a severe infection (sepsis) or severe respiratory failure (acute respiratory distress syndrome).

This is a clinical trial for patients with neuroblastoma that has returned (relapsed) or has not responded to standard treatment. The patients will have a biopsy of their tumor that will be tested for genetic changes and then matched to available drugs. If the results from the testing show molecular features in the tumor that allow for study treatment, patients will be invited for further testing to see if they are eligible for new treatments. Experimental therapies are all taken orally and may consist of one drug or a combination of drugs

The purpose of the Phase 2 portion of this study is to evaluate the safety and efficacy of the study drug called palbociclib when given in combination with two other drugs (temozolomide and irinotecan) in patients with Ewing sarcoma and when given in combination with topotecan and cyclophosphamide in patients with neuroblastoma.

This study involves taking a study drug called selpercatinib. The overall goal of this study is to find out how effective the study drug is in combination with radioactive iodine (I-131) in treating people with thyroid cancer. The study drug will be given in combination with treatments you will receive as part of your regular cancer care (I-131 and thyrotropin alfa).