We went for the first time this year and decided to go to de Eemhof in The Netherlands with 3 children (8y, 6y, 18m). Took the ferry from Harwich which was excellent and the children loved the adventure (we paid a bit extra for the lounge but it made such a difference to the journey). Travelling through Holland was a doddle too.
We had a kindegarten cottage which had everything you could want with children - a play bedroom with a puppet theatre, a chalkboard, trolley of toys, highchair, cot and for babies a bath, nappy bucket, bouncy chair, bottle warmer, potty and training seat, a step, cutlery, etc. It was clean and comfortable - a change of bedding was provided as well as towels. The decor could have been freshened up but with three children you don't really worry too much about it. They were very efficient with any problems (new frying pan immediately, extra towels delivered). Obviously they spoke superb English so communication was easy.
Plenty of places to eat and "children eat free" at one of them each night. Supermarket very good and reasonable - we took nothing with us and spent less than we normally do in England!
The pool complex was very good although they seem less concerned about safety there - you have to watch out for the odd rough bits. Older children thought slides and flumes were superb and the baby loved the two toddler pools (one with a sandpit in the middle).
Cycling is amazing. They have everything you could want even a family cycle to hire by the hour. The cycle paths outside the park are great and there's a particularly good one along the dyke (literally on the edge of the park).
There are plenty of things available to do but we were more than happy with the pool, cycling and the little play parks just by the cottages. The cottages were very close to the centre too.
For a day out there are quite a few things locally - amusement park, zoo, dolphinarium but Amsterdam is only 30 mins by train so we did that. Children loved ( gay floating carnival was on)
Sorry this is so long but it really was great - we could easily have stayed longer (weather was perfect).