Hi Sarah
As you already know, it's not compulsory at that age but our school in Manilva has created an extra class this year for "infantil" so it is possible that they can do that in La Cala too. I assume it's relative to how much pressure is applied on the school, but not necessarily so as the Directores have very little say in Spanish schools.
I would definitely go in there as often as possible. It's probably unlikely that kids will be dropping out but you never know. There are quite a few expats leaving so some places may become available.
It's a tough one. Your only option may really be a private school. Check out the private Spanish schools, not international ones, as they tend to be cheaper and everything is in Spanish. Saying that, some of the Spanish private schools we know are also quite full and they too have waiting lists.
Best of luck with it, we were where you are now six years ago, so I feel your pain!
Justin