Each country has their own requirements for minors
France
Spain
Italy
Italian minors under 14 years travelling with the new style ID Card “Carta d‘identità” and with one or both parents can travel on EU/Schengen flights with no additional documentation required only if the name of the parents are mentioned in the document, otherwise, the parent or the legal guardian must carry the family status certificate or the birth certificate of the minor. If the minor under 14 is travelling with the new style ID card with neither a parent nor a legal guardian but with a third person, he/she should be in possession of an “affido“signed by both parents or the legal guardian and stamped by the Authority in charge of issuing passports.
Portugal
Portuguese nationals and alien residents under 18 years leaving or re-entering Portugal, to or from a non-Schengen member state unaccompanied by their father, mother or legal guardian need a travel authorisation. This must be signed by both parents or legal guardian with the signature notarised if the parents or legal guardian reside in Portugal; or authenticated by a Portuguese embassy or consulate in the country where the parents or legal guardian reside. This travel authorisation is also required when minors are accompanied by a person other than their father, mother or legal guardian. In such cases the travel authorisation must also clearly show the name of the accompanying person. Foreign minors under 18 years and traveling alone may be refused entry if they do not have anyone in Portugal taking responsibility for their stay.