For a family of four, ideally nice rural touring, with the possibility of a campsite for a few days (upto a week) that has good resources, pool slide etc fir youngest. Lakes and hills ideal. Looking at around 2 weeks total give or take a couple of days.

Thanks

Gerardmare in the Vosges.

went with the kids one year following the tour.
lakes with swimming park in it, hill galore, dry skiing, summer luge, climbing walls.
nice medieval towns galore with ace food.

Timbuktu?

Gerardamere a really good shout. But possibly too hilly for the kids?
How about train from Amsterdam to Den Helder, then the ferry to Texel? Amazing cyclepath network, lots of dunes and nature, and you can take boats to the next few islands easily and cheaply.