Hi Steve
I don't know the area but most schools I think work the same.
The official schooling age is 6 years old. Before that, schools aren't obliged to take in kids (it nearly happened to my kids) but I think most schools today do an infant intake at 3 years old.
The best thing to do is go straight to the school and see the "director" (headmaster), which is what we did, and he'll advise how to get the kids into the school ASAP.
Sometimes if you go via the town hall they just fob you off with stories that there aren't any available places, which is why you need to go in there direct.
I think you will need to be on the padron in order to get the kids enrolled.
Hope that helps.
By the way, we have an ebook on the site here all about schools in Spain which you may possibly find of interest to read.
Best of luck with it all and let us know how you get on.
Justin