It's an increasingly common situation; children leaving Spain to go away without parents, on a school trip or with friends of the family. With the rise in the risk to child safety, Spanish authorities have announced that from 1 September, the rules for kids up to 18 travelling this way will be tougher, and this could affect some expat families.
From now on, a child on a Spanish passport or national ID card (DNI valid for travel in the EU) will also have to carry signed authorisation from both parents on a form administered by either a National Police or Guadria Civil station. Previously this only applied if the child was using their DNI and not a passport.
If the child is travelling on a foreign passport, but is resident in Spain, official advice from the Spanish authorities is to check with the consulate of the child's citizenship what the procedure is for that country's citizens, although the rules vary by country. For example, in the case of the UK, a parental letter is usually considered sufficient in more straightforward cases. There is no requirement to fill in a form and the British Consulate cannot provide certification. The UK Embassy advises that from 1 September, underage UK children leaving Spain, "may need written authorisation by a parent or legal guardian if ...travelling unaccompanied, with a third party or other circumstances".
The toughening up of the rules doesn't affect people who have just come on holiday to Spain and who aren't resident. Here, people responsible for children travelling alone need to comply with the rules in their country of origin.
For example, official UK advice for children travelling from Britain abroad is, " A letter from the person with parental responsibility for the child is usually enough to show you've got permission to take them abroad." This should include details of the trip. It adds that in some cases a court order to travel is required.