Have also been to Duinrell (twice and will definately go back again)
Love it there, and if you have a car it is so easy to get to and get to Amsterdam and other places as well.
Loads to do in Amsterdam, buy Holland passes from the Travelex shop in the train station, you get free entry into loads of places and discounts galore!
If you plan on vising a 3 or 4 attractions in Amsterdam alone, you will save a fortune. The Nemo Science Museum is totally worth a visit and if the weather is nice, make sure you go up to the top for a coffee, relax a bit and dip your feet in the paddling pool thing they have!
Mind you. you could spend your whole holiday in Duinrell and the kids willl love it.
We stay with Keycamp and the caravan has always been lovely and clean.
My only "complaint" was that there was no TV, so the second time we went we took DD's small tv from her bedroom and some DVD's for night time.
The pool there is fantasic and the funfair is pretty good as well, will definatley keep your 2 boys entertained!