Do not, whatever you do, go to Haggerston Castle.
We went there last year with my parents, DD 5yo and DS 3yo.
There is a big amusement arcade, through which you have to go to get to all the facilities, even the playarea.
Only one playarea for the whole place.
The pool was nothing special.
All the activities were extras, some of it very expensive. The kids club was a complete rip off, they sold some kind of badges or medals each evening.
Where abouts are you? You can get some great bargains on the Dutch Centerparcs if you leave it till the last minute. Depends on when you are going too, if the German/Dutch schools are off you might not get anything.
We went to one in Holland recently, weekend for 150euros for 5 bed bungalow.