Between Member States of the European Union (EU), minors traveling alone, in the company of adults who do not have their legal custody or only in the company of one of the parents will have to carry a valid ID or passport, as well as an official document authorizing the trip signed by parents, legal guardians or the other parent. You have all the information in: «The Schengen area and the Member States of the European Union«.
You have all the information in: "The Schengen area and the Member States of the European Union".