I just wanted to add my experience in case it helps. (Maybe too late) We moved from the USA to Spain, San Javier- Murcia, with a 12 and 5 year old. The decision was to put the 5 year old in the normal Spanish school system and he had a nice Pre-K school setup nearby. He learned quickly enough to make himself understood, within the first few months.
The older one, was placed at the near by private, (fee paying) Castelar International school. It was a very disapointing experience. The vast majority are local kids that are either family related or long time friends of local surrounding communities. The school, after the economical crisis, had an important withdrawal of registered students and it is now suffering financial difficulties. Pool has not been opened the entire year, no tenis or other sport facilities outside hall and football field. If your kids are not into football they will soon find themselves left for last in the playground field. Teachers are higher ratio Spanish speakers, some with limited English. Subjects are in both languages but you will have subjects like math divided into two complimentary areas one been in English and the other in Spanish. Everyone has to atttend both. If your are not up to speed in Spanish, you will have problems following the overhall course material. Because of their financial difficulties, they do not have much to offer for extra curriculum or teacher's aid to take someone appart and focus on Spanish. No Psychologist on board to help kids with integration problems or counseilling. Teachers are given a sheat sheet by an external Psychologist, in case of need to help a student.
Since you are required to have a certain level of Spanish by school end, to pass Spansh education requirements, you will have to pay and arrange other ressources to privately teach Spanish to your kids.
With this experience, I will recommend that:
* You look for well attended and funded schools. Look specially for the offer in subjecst like sports, art, computers, music, etc. These are subjects first eliminated in case of financial problems or you will find that the same teacher who teaches sciences, teaches english, computer and other subjects requiring specialization.
* Look for an English/British school with curriculum in English and Spanish offered also at beginer's level, even if the student enters 5th grade.
* Be prepared to pay around euro 500-600 per month, plus registration fees, plus uniform, plus transportation.