Thankyou for all the replies and I am glad if this thread helps others decide the best course for them also.
there is immense pressure not to make our children the 'odd one out' in every aspect of their lives, and as such it's really hard to do something that goes against the flow. I wish more people did the part time thing - it might not seem such a bad thing to do for our own children, if it was more commonplace.
For me the idea of ds2 struggling as much as ds1 did is unthinkable so I will have no problem deciding to take him out or do part time should it be right for him. I feel so sad seeing some of my friends' little tinies crying and exhausted, and their parents under such pressure to send them so they don't miss out. I feel that our system is thoughtless in demanding so much of children who are often only just past three years old.
I think usually they make a successful transition, ie adjust to it, during the first year, but having seen the enormous difference in stamina and sheer coping ability in my first child between the ages of just four and just five, I don't see the point in forcing this huge adjustment before they are physically, emotionally and mentally ready - it isn't a question of physical activity being forced on them, but the processing of all the changes and rules and instructions and new faces, which at that age is often really hard work.
They come home with their little brains full of new things and have to take ages to wind down and sift through it all - a bit like we do after a big day out or a big party, we can't just switch off, we are buzzing, yet at the same time very very tired.
SI think it is really important that they are allowed this short year which carries so great a development, such a change in them, if they need it, in order to develop at their own pace just as they have done all their lives from when they were babies. Forcing a change they are not ready for is not usually a good idea at any stage.