Norsk Folkemuseum is one of the biggest and oldest open-air museums in the entire world.
Actually, a "museum" sounds way too dull for a place like this. Except for the hundreds of traditional Norse houses and artifacts, the Norsk Folkemuseum hosts numerous activities especially interesting for the children. They include thematic exhibitions of folk costumes, toys, handicraft items, etc.
In the summertime, things get even better with the outdoor horse riding, workshops, pancake factory, and guided tours.
The entrance is free for children under the age of six.