John,
I work in Rotterdam (and the UK) and Dutch people are great, so I'm sure you'll have a great time.
Advice:
1. IJmuiden to Wassenaar (Duinrell) will take about 1 hour - remember the maximum towing speed in Holland is 80 kph (50 mph) on the motorway - and this area between Amsterdam and Rotterdam is quite busy during the rush hours
2. Eating out (with children) - all Dutch restaurants and cafes are child freindly - most Dutch families eat out on a regular basis
There's no need to eat formal meals in restaurants or cafes - just order what you want - if you only want a starter and a desert then that's fine
Tip - the normal in Holland is 5%
3. If you can drag your kids away from the water-park - a day trip to Delft is worth it - it's only about 10 km from Duinrell - as is a visit to most Dutch cities.
If your children are quite young, they might like a day trip to "Efteling" (nearest town is Tilburg) - the park is built up around European fairy tales - and as such is probably not suited to cynical teenagers
4. It is illegal in Holland to simply loop the caravan breakaway cable over the tow-ball, you MUST attach the break way cable to a fixed point on the tow-bar itself - Dutch police have been checking cars and caravans at the ferry ports - and issuing fixed penalty Euro 30 fines.
5. Most Dutch people under the age of 40 speak really very good English - but if you do learn a few Dutch phrases then they really appreciate it (especially if you manage to mangle the pronunciation) - learn "Dank U" (Thank you) and "Austublieft" (Please) which is used to attract waiters, bar tenders, receptionists, etc., you'll hear these used all the time
6. Credit Cards - most shops will happily take UK credit cards - except for the railway stations and parking meters (and a few other places - like some supermarkets) who only take cash or the special Dutch chip-and-pin cards.
Although they have what you think is the standard chip-and-pin sign, these Dutch cards work differently - they are an electronic version of cash - a Dutch person will go to a cash-point and transfer some money onto that card - then if they loose the card (or if it is stolen) - then they only loose the few Euro on the card.
In any case, it's incompatible with every other chip and pin card in Europe - but if the sign says Visa then you're OK
7. I work for a company in Spaanse Polder (facing Rotterdam airport across the A13 motorway) - just South of Delft - if you need any local information - let me know
Robert