Remember many parts of Spain are bilingual, so the kids will be learning their own language + Spanish (Castillian).
My guess would be a choice between French and English. Many people I know of my age or younger speak excellent English, but Spanish people in general don't speak as much English as I've found in other European countries. Nothing wrong with that - their language is spoken throughout the world after all!
I think portugese is an unlikely answer, as Portugese really isn't very different from Spanish. I certainly found that even in the very south of Spain, people spoke French. And train stations had signs in French and Spanish.
Depends where you are, but at the school near my dad (mallorca) they have their main lessons in Catalan, learn Spanish too (like the Welsh-medium schools here) and then have a choice of French/English/German.
I've received quite a bit of interest from prospective Spanish au pairs recently, and they've all claimed to have either a good or basic knowledge of both French and English.
I don't think it's totally fair to compare English speakers with non-English speakers. We have a lot less exposure to foreign languges than other Europeans have to English/American.
I'm 40 and can speak French - was actually able to use my school French to work in a factory in France between the ages of 23 and 26. Were I applying for an au pair position, I would have happily declared that I could speak half-decent French.
Schools round here teach English as a second language. Sometimes they often French too, but usually just English. The local language academy has about 15 English classes and 2 French ones. No German or portuguese, although portuguese might be more common near the border.
I've read that in parts of Cataluña, some children don't even learn Spanish - their main lessons are in Catalan and they choose English (rather than Spanish) as a second language