The purpose of this study is to test the safety and efficacy of the investigational drug called Selpercatinib in children whose cancers have a specific genetic change (RET-fusion or RET-mutation). Selpercatinib has been FDA-approved for adults and pediatric patients 12 years of age or older for certain types of lung and thyroid cancers. Selpercatinib is administered in capsule form, liquid suspension, or may also be administered by a feeding tube for subjects who are unable to swallow.