With a 4 and 6 year old in Maastricht, why would you not choose instead a local no fees Dutch medium school where in any case they learn English in primary and you're probably likely to be quite popular if English is your home language. Netherlands is one of the easiest countries to integrate English speaking children into local schools.
I think s999 is in Singapore and is asking about tampines or Dover campuses. We used to be on uwc waiting list and they were very helpful about answering questions about whether (or not) we were moving up. Can you contact them?
Which UWC are you asking about? There are twelve UWC schools and colleges around the world. They are in Canada, Hong Kong, India, Italy, Norway, Singapore, Swaziland, the United States, the UK, Costa Rica, Bosnia and Herzegovina and Maastricht.