Taking into account each case, respectively if the minor has Spanish citizenship, dual citizenship, if he is accompanied by both parents, guardians, etc., you should consult www.guardiacivil.es
For crossing the Romanian border, the provisions of the Romanian authorities, www.politiadefrontiera.ro
Notes:
• Please note that neither the Family Book ("Libro de Familia") nor the birth certificate are NOT valid travel documents.
• It is advisable to consult the transport company, which may request additional documents.