We stayed at a campsite with lots to do for children, I think near Appeldoorn as someone mentioned above.
There is one campsite I would say to avoid, just in case you find yourself on the border or Germany (we were only in that area because we were going to Germany). It is near Enschede. It's used by Germans partying at the weekends and during the week it is empty and the rats are hungry. Worst camping ever.
But the one near Appeldoorn was lovely. Just be aware that the weather over there is often rainy.
The best camp sites will have had a high score from the Dutch AA (ANWB). Some even use the title "kindercamping" which means child focused campsite.
http://www.anwbcamping.nl/kindercamping.html and scroll down for a list of campsites and scores.
The campsites with a high score (no knowledge of Dutch needed to understand a scoring system) all have very good facilities, which includes sanitary facilities (great showers!) shopping facilities and leisure facilities.
I would first decide what you want to do and then look at campsites.
For children I would highly recommend:
The Efteling - a must, much bigger that Disneyland Paris, you can easily spend two days there. There is an Efteling camping but there are also cheaper campings nearby. One is a mini camping at a farm, basic facilities but short walk to the Efteling and the centre of Kaatsheuvel for shopping etc.
Verkeerspark Assen - little pedal cars and they can get their driving licence. It is brilliant, good food, nice playgrounds and they can spend hours in the cars as it is very big. Camping Witterzomer is close by, has lots and lots of facilities, lots of kids activities as well.
You can make a trip to see windmills (they are all over the country, but Kinderdijk is probably most famous, but also expensive and touristy).
In Lelystad is a nice museum which explains Dutch waterengineering, dyke building etc.
Depending on when you go, campings might be full, so it is worth booking in advance if you go during July/August.
Many Dutch shops won't accept credit cards by the way, it is not like the UK. You will need access to cash as they may also not always accept UK debit cards.
We love the Hoge Veluwe area (between Appeldoorn and Arnhem). There are loads of campsites in the Beekbergen / Hoenderloo sort of area. Things to do - Apenheul (monkey zoo with free-ranging monkeys), steam train, Open Air museum in Arnhem , Hoge Veluwe National Park (cycle round on the free white bikes), Paleis Het Loo (Royal Palace in Appeldoorn).
Amsterdam is obviously well worth a visit but you would probably need to camp outside Amsterdam itself and get the train in. There is one campsite I know of in Amsterdam itself but it is aimed at backpackers and may not be the best place for families.
If you visit a lot of museum / galleries, the Museumkaart (museum card) might be worth getting - it gives access to about 400 museums and galleries nationally. There are also a couple of Amsterdam visitor cards which allow reduced entry to Amsterdam attractions - try googling if you don't get any more info here.
We've never needed a camping card. The campsites we have been to have been generally very nice, flat obviously, and sandy so easy for pegging. I don't remember ever seeing kitchen facilities other than washing up stations. They don't seem especially cheap though, around 30 euros a night for a tent and family of four.
And Dutch playparks are fabulous. They often have sand and water pumps with log channels so lots of wet sandy play!
We are hoping to do a 3 week trip to Holland next summer. Very excited, and I want to get some ideas and plans in place to help me through the dark days of winter.
We will most likely take the ferry over from Harwich, probably overnight both ways. What's the deal with onboard food? Happy to book if good value and served at helpful times, butt he ferry leaves late and arrives early, so are you allowed to board before?
What places would you recommend to visit? We are quite keen to have 4-5 bases in that time and are happy to travel all over. We like a mixture of things, children will be aged 9,7 and 4.
What are the campsites like in terms of facilities? Do any of them have kitchen facilities? What should we expect in terms of prices? Do we need a camping card?
Anything else that we need to know? So excited, have I mentioned that?