Ah, many happy memories of Duinrell when the kids were little. Always a success.
I haven't been for a few years, so my advice may be out of date, but we preferred booking direct and getting a 'Dungalow'. The check-in process was quicker and the accommodation was better situated and just nicer and Eurocamp seemed to have so many rules and regulations - (having some 15-year-rep old checking the inventory and judge how well you had cleaned at the end of the stay really put a dampner on things!)
When we went, the amusement park was quite tame - perfect for little ones, not so much for thrill seekers. It was nice to be able to do it in chunks, so go back to favourite rides etc. There is also a nice playground and some jolly shows etc.
They may have changed it, but booking direct meant pool entry was at certain times (like 10 til 2 and after that you had to pay - or it was open to paying customers? They may have changed that, though).
You will need to get out at times to avoid cabin fever. The little town, Wassenaar, is a lovely stroll (there is bike and wagon hire at Duinrell, which is fun). The beach is good. There is a really fab natural history museum at Leiden.
Anyway, bottom line is we really liked it and went about 3 times. That was enough I think.
(Also, my crappy flip-flops got nicked from the changing rooms - didn't think they were worth putting in the lockers, but someone obviously saw an opportunity, so yeah, use the lockers!)