Just to clarify, the Cito eindtoets takes place during the second term in group 8 (February, over a three day period, it is a very extensive test).
It is incorrect to think that the Cito toets takes place over a two year period. Some schools take the Cito entree toets in group 7. However, in order to decide to which secondary school a child goes it is the eindtoets which matter.
The entree toets is simply part of het leerling volg systeem (which tracks pupils achievements over time). Many schools use Cito toetsen in other years as well to track achievements.
There is lots of testing in the Dutch system, but the crucial one is the Cito eindtoets. A top score will open many doors, a poor score can be very problematic. Again, I would like to stress that it is a tough test if you want to end up in the top as many Dutch parents will carefully prepare their kids for the Cito toets.
I remain of the view that you are going to ask a lot of your nine year old to settle in in a very different environment, make new friends, learn Dutch, learn new subjects and perform at his best in the Cito eindtoets. However, it is also not my experience that kids are fluent Dutch in 6 months, in my experience this takes a lot longer. There is a big difference in my view between being proficient and fluent. A proficient child is no match for a fluent child. A proficient child is not going to score as well on the taal toets and definitely not in the top, which is required to get in one of the best VWOs. A proficient child needs more time in the other tests compared with a fluent child and hence will be at a disadvantage. Even if he knows all the answers it is unlikely he will be able to show this as he is unlikely to be able to complete the full test in the allocated time.
If you really feel very strongly that you want to enrol him in a Dutch school straight away, I would at least:
- get a good private tutor for him
- explore with the school if there is any way in which they could get a dispensation for him so het gets extra time when he sits his Cito eind toets.
I teach in a UK primary school but also teach Dutch kids (who speak Dutch at home) who are returning to the Netherlands to complete their education in the Netherlands. These kids tend to be proficient, not fluent, in spite of being taught Dutch from birth and speaking Dutch at home. We do lots and lots of extra work with them to enable them to get a good result in the Cito eindtoets. So based on my experience your nine year old will have a few very tough years ahead of him. For the seven year old it should be much easier and for the younger one no problem at all.
Good luck.