EU Citizens do not need a visa to au pair in Spain. EU citizens have all health costs covered by the Social Health Security of Spain. The students must have obtained an E-111 form, before leaving their country, which will enable them to enter the Social Security System of Spain.
Non-EU citizens have limited options. If you are going to Spain for less than 90-days, you can go on your passport to visit a host family. The Student visa program in Spain now requires you to be a full time student. If you are applying for a full-time Student visa and plan to stay in Spain longer than 6 months, US citizens must obtain an FBI Background Check, including fingerprinting, that is notarized and legalized by the "Apostille of the Hague". If you are from outside of the US, then you must present Certificate(s) by Police authorities from ALL PLACES where you resided for more than 6 months, during the last five years. Please contact the Spanish consulate in your area for specific details. US citizens can find detailed information at: http://www.consulate-spain-chicago.com/student.htm
There is conflicting information on the internet about obtaining a Residence permit in Spain. Contact the local Spanish embassy or consulates office to see if your host family can sponsor you for a dependent residence permit during your stay. If the residence visa is granted, you may travel to Spain where you will then have to apply for the residence permit, which involves three visits to the police station, over a period of about 1½ - 2 months, until you obtain your final residence permit.
There is additional information on the Spain embassy website at: http://www.maec.es/, that defines their National visa as follows:
These visas allow the holder to reside; reside and work; study; or conduct research in Spain. All foreign nationals who wish to enter Spain in order to reside; reside and work; or study must be in possession of this type of visa, unless they are citizens of the European Union, Iceland, Liechtenstein, Norway or Switzerland.
Aupairs cannot work more than 30 hours per week including evening babysitting and will receive pocket money of 50-70 Euros per week