We spent a week there at the beginning of July with 3 kids (11,8,5). Had a brilliant time, we didn't go every day to the pool. We just went to the amusement park, went on walks, went into Wassenaar, went to Amsterdam for the day, the whole week was really relaxing. We were staying in a cabin / bungalow.
Wifi is poor in the accommodation, we knew that in advance so had downloaded a couple of films for the kids. Weather was alright when we were there - it rained a few nights, a couple of heavy showers on 2 days but that was it. The accommodation was really well kitted out re crockery, cutlery, utensils but we didn't cook once self catering isn't my idea of a holiday, I booked this one under duress. We had breakfast, did something, picked up rolls, meat picnicy stuff, then dandered into the town for dinner. Some days we just snacked our way through the day - it was brilliant.
We went to the Jumbo supermarket in town a few times they do 3 tapas based containers for €5 - buy more than you ever think you'll eat because you will eat them 
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Go carts, actual play park beside the amusement park centre, swing chairs with water jets, rollercoasters, Carousel inside the pancake house were huge hits with my three.
If it rains there is shelter in the amusement park, some of the attractions are covered - there are bumper cars, an indoor soft play, the big wheel we did a few times, including once in a drizzle.
Just make sure you've an anorak that dries quickly, shoes that dry out well and you'll be grand. We flew so we didn't have that much room for extra stuff but with a car you can always stick something else in.
Hope you have a brilliant time, like I said it was a different sort of holiday for us, but we loved it, the girls loved it and we are already thinking about going back.