Should summer eventually decide to hit Northern Germany this year a must-do with children is a Wattwanderung, and be it a very small one! There is nothing more fun to a mud-addicted child than this. The best place is Büsum – a small touristic fisher village at the North Sea coast. Check the tides before going, because if you arrive there during the flood you won’t be walking the mud at all (http://de.windfinder.com/tide/buesum).
There is also the museum Blanker Hans, should one still feel kind of underemployed (which is actually quite unlikely). Still worth mentioning. The museum on storm floods and pirates is worth a visit.