So we (dh and I and our 2 sons aged 8 and 10) are in the fortunate position of having the whole summer off and are planning a 4 week trip to the countries above. We will be getting the ferry to and from Netherlands from the UK at the end of July and have 4 weeks before finishing with a week in Center Parcs in Belgium at the end of August (booked ages ago before we knew this extended trip would be possible).
We will mostly be camping but want to plan a few nights in hotels / self catering accommodation. Budget is basically as cheap as reasonably possible, but this is a once in a lifetime trip made possible because of a combination of circumstances that won't be repeated. So we can afford to, and would want to, push the budget for something special.
If the DSs are into Legoland, you could visit Danish Legoland in Billund.
There's a Centerparcs style place next door and an indoor waterpark/entertainment complex.
The coast about an hour west of Billund is nice with beaches/nature reserves etc and I think Copenhagen/Malmo is about 3 hours to the east, so you could do those in a logical order to fit in with everything else.
There's a big festival on in Malmo in August, (we'll be there for a couple of nights then) I think most hotels are booked, but camping I guess you'll be fine. We're down in Southern Sweden too - never been before, but have been recommended to go to: Ystad, Simrishamn, Kivik, Skillinge, Bornholm island, Ale Stenar, Glimmingehus, Knabackshusen beach. Will also be in Copenhagen - there we've been told to go to Tivoli and Louisiana modern art gallery, which looks fab. I'm v jealous of your month-long trip!
Tips for Holland... Den Bosch is meant to be a very typical beautiful Dutch village, Maastricht is also a popular city destination. I've just been to the 'venice of Holland' in Giethoorn, amazing. No cars allowed- only bikes or boats. (google it!!)Plenty of camping sites around there. Also if you can, try and visit the Islands, I believe one of their beaches was voted in the top10 of the World (Ameland I think??). Tips for North Germany...we've been to Dusseldorf, Cologne (Koln) and Munster, all really beautiful cities so any of those are definitely worth a visit.