Travel documents for minors – Spain
Children require a valid travel document to leave Spain. In addition, if the children are travelling with an ID card rather than a passport, they must have written permission, signed either by both parents or the parent/person who has custody, and certified in any police station, civil guard station, court, notary or city hall upon presentation of the family book or birth certificates.
You can find an example of this form on thees website.
Minors from an EU country entering Spain with only an ID card (and not a passport) also require written parental permission.
Contact (to enter Spain)
Contact (to leave Spain)