It's not quite as easy for him to enter the UK from the Netherlands as it is travelling to the neighbouring European countries in any other direction on the compass - the UK has immigration/border controls and he must show acceptable identification on entering the UK. Going to Belgium, Luxembourg, Germany, Denmark, Norway or even Iceland there isn't any border control, no immigration, etc. and he really can go there as if it were the next town. The UK is the odd one out....again. In all of these countries he can seek employment and live there as long as he chooses - unless and until the UK leaves the EU in which case the UK becomes a bit of a no-go area. Conversely your daughter (and anyone else) must show identification on entering the countries mentioned from the UK (the UK is not part of the borderless system) - and the UK authorities may even ask her for it as she leaves the UK. From there, however, your also daughter can enter all the other countries without being required to show any identification (it's called the Schengen area, after the treaty).