You can also use Camping Cheques (www.campingcheque.co.uk) at Easter in Holland, and there are several sites which look really nice. There's one at Koningshof with an indoor pool which looks just the sort of thing kids would love. It's 5 kilometres from the beach, but at Easter it's not going to be warm enough to sit on a beach all day, but a trip out would be a nice option. At Easter weekend you'd pay normal rates, but the remainder of the holiday would be at Camping Cheque rate. They also organise activities during the Easter period, as well as a coach trip to Amsterdam. Could be just the ticket????
Duinrell seems to be a recommended park for families in Holland - with a theme park and water park next door, and discounts I think for people staying on the campsite.
I haven't been to either of these sites, but perhaps someone who has can add a bit more to my descriptions.