Have been to Duinrell and De Kempervennen. Both excellent, and both had a resounding thumbs up from the DCs.
Duinrell we went to when they were primary-aged. It has a great theme park (mini-Chessington) and massive waterpark with some fiendish slides. The usual bike/go-kart hire and whatnot, and it's close to the coast.
De Kempervennen we went to last year, when they were all teens - much better, I think. It is huge, with two lakes, and just about every water activity you could think of - waterskiing, scubadiving, kayaking - also, there are things like high-ropes, zip wires, an indoor ski centre with proper snow, a mini-zoo.....honestly, it's fab. Pick your type of house carefully - we don't like anything basic, so the one we chose was amazing - overlooking the lake, with three bedrooms, a sauna, posh kitchen. Before you go, you can book various activities, hire bikes (essential!) or select bundles of restaurant meals / morning bread delivery to your door / BBQ kits.
Getting there is simple; both are a fairly straightforward drive from Calais, or if you can stretch to it, get the sleeper ferry from Harwich to Hook of Holland, and you'll be 30 minutes away.
Writing this makes me want to go again - it was brilliant.