please help: state or private prep in wimbledon

[yeahrush]

would like my DS to WHS or KJS by age of 7.

which prep schools should be the best to get DS into KJS or WHS? state school like dundonald or private school like Donhead, Willington or squirrels? please give me some advise! Thank you!!!

WHS is girls only so you'd better cross that off your list.
It's tricky to get into most state primaries unless you live really close to them, and that's especially true for Dundonald. Bear in mind that state primaries will not prepare your ds for 7+ entry.
Schools that go through to 11 or 13 such as Willington will also not prepare for 7+ as they do not want to lose pupils.
You are best off with a known feeder school such as Squirrels or Rowans. How old is your ds? Is he showing signs of academic prowess? KCS is a highly academic school and very oversubscribed. You have to be very sure if you want to embark on a system where you have to leave a school at 7, because there aren't many local alternatives which have a 7+ entry.

If you want kings at 7 wouldn't look at state. You would really only be thinking of the kings feeders . I would keep an open mind though. Kings feeders don't guarantee a place at kings and then you are looking elsewhere at 7 . If you want to stay local you will then have to hope for a year 3 opening.

If you want to move him at 7 I would only consider squirrels or Rowans. If you would consider moving him at 13 consider Rokeby or Homefield.
My son is at the latter and it currently has the best results at KCS locally. It's in Sutton but regular buses from Wimbledon.

Good luck