Sognsvann lake recreation area is another excellent place to take your children when visiting Oslo. The venue is out of town, but it's just 7km (or about 20 minutes) away if you take the Line 5 subway or drive to the spot.
The lake is fully surrounded by a walking or running path which is 3km long. All around the lake, there are also plenty of beautiful places for picnics, fishing, swimming, and different family activities.