They don't offer special school places unless they really need them, as they are like gold dust, and they don't keep them there unless they really need it, well this is what it is like within our LEA. If they are performing academically close to their chronological age, they will have to go back into mainstream, so it frees up the place for a child who really needs it.
Your dd could go for a couple of years to SS if they are making excellent progress and catching up with her mainstream peers, than you can move her later on. It sounds as though, right now, this is what she needs. The special school that ds hopefully will go to in September, is anything but keeping them happy. It has an outstanding Ofstead report, and when I looked around with dh, the teachers were passionate about their teaching and happy. For example we were shown a Yr4 classroom, they were learning about the human skeleton and the different bones, I was so impressed, in a Yr7 class, in RE they were learning about the Torah, Bible and Quran.
There were 3 tiers sensory, structured and scaffolded, to reflect pupils different abilities, sensory being, those who are more lower functioning, structured-those who were in the middle, and scaffolded-those who were very able academically. In their secondary school department, they offer a range of vocational qualifications and work placements, and are expanding their GCSEs. So not all special schools 'keep them happy'.
I really hope ds will go there, as he will fly, and do so well. The gap will widen in mainstream school, and his mainstream peers will be going at a far faster pace which will just affect his confidence. With the funding crisis now in mainstream schools for children with SEN, it is a good time for ds to go to SS.