The deadlines for school registering are very misleading - they really do not exist! It is the same as the UK. once you have your padron etc. your children are entilted to go to school and receive an education.
If you are moving into the area (from the UK, Madrid, Rome, Paris etc.) your child is entilted to an education. I have heard stories from people saying that if they do not register in May for Septemberr intake they cannot get into any school until the following September - absolute rubbish!!! Do you really think that a Spanish family who move beteween towns / cities within Spain would allow this? Think about it if you moved from London to Manchester would you wait months for you children to start a new school - I don't think so. The same rules apply in Spain. If in doubt get a translator and go and see the local Mayor.
If you want both of your children in the same school then again enlist the help of the Mayor (and tranlsator if needed). Children welfare and education are of a lot higher priority in Spain than the UK do not be fobbed off by rumours, school receptionists or even head teachers.
Simple advice - do NOT listen to rumours, peoples opinions (even mine) until you have fully investigated through official channels. We heard all of these rumours and decided to go and investigate and ask probing questions ourselves. We based a lot of the questions on what we would do in the UK if we moved location - guess what the rumours simply did not stack up!!