Spanish minors under the age of 14 are exempted from proving their identity, for domestic flights in Spain where their identity is stated in the reservation. In such cases, the person who is traveling should duly identified and should take responsibility for the minors. In the event of reasonable doubt regarding the identity of these minors or of their companions, the following is requested:
If they travel with one of their parents, the family book.
If the person who accompanies the minor is not one of the parents or legal guardian, the minor should show a permit certified by a police department (by means of the appearance of the parents or legal guardian) in the name of the person who will be responsible for their custody during the trip, which will be corroborated with this person's National ID or passport.
For international flights (EU or Other Countries), all minors (whether accompanied or not and regardless of their age) should show their National ID or valid passport, depending on their destination.
Starting September 1, 2019: Spanish authorities will require that all Spanish minors travelling alone or accompanied by third parties who are not parents or guardians have a travel permit issued by one of the following: Police, Civil Guard, Notary's Office or Town Hall, requested in person by the parents or legal guardians.
For more details, please visit this page.