Hello Southwest
I have lived in Madrid for 6 years. What you have to bear in mind about Spain is that there are many regional differences so a lot depends on where you go. In Barcelona for example all schools teach in Catalan so you have two languages to consider, the same goes with schools in Valencia and the Basque country.
Not all schools finish at 2pm, again depending on where you go you may find schools have a split day whereby they have lessons 9-1pm a 2 hour lunch break and then lessons again from 3 until 4.30 or even 5pm.
As somebody mentioned up thread school applications are made in April to start in September and the school year is a calendar one Jan thru to Dec so depending when your DC were born they could be up to a full year behind or infront iyswim. DS1 is December born and so the youngest in his class. I only have experience of the 1st year Primary so far, but it´s true that I find it to be quite an old-fashioned system compared to the UK. Lots of copying and dictation and homework already and DS is only 6. This could be a disadvantage for somebody who is new to Spanish.
If you apply for a school place outside the official period you are likely to be offered the school that has places whether it´s one of your choices or not, it may not even be near to your house. Just to be clear, I am talking about Spanish state schools, I can´t talk for the private ones, I know they have a different criteria.
As you know already, Summers are very hot and dry (up to 40 in Central Spain and the South) and even in the North of Spain Summers get far hotter than what we are used to in the UK, much more than just a few degrees
If you don´t like the heat it can be unbearable and it can be hard to find things to do with small children. I speak from bitter experience.
There are plenty of Summer camps even for small children, in fact in my experience most parents send their children to these sorts of activities in the Summer from a very young age. I don´t have any personal experience though.
Good Luck. Honestly...I think it can be very hard to break into Spain, I have struggled and I live in a big city. In my experience Spanish people tend to stick with the friendships they have had for years,and rely on their family more, most people work very long hours and the parks are usually deserted during the day. As already mentioned, times in Spain are tough at the moment and the mood is fairly low, I am not here out of choice.