On bus journeys during Saturdays, Sundays, and public holidays (with the exception of May 17th), you, as an adult or senior, can bring up to four children under 18 years of age for free, as long as the travel distance is the same and within one zone.
You can purchase a regular single ticket in the app and scan the ticket as usual, or buy it on board from the driver (note that this incurs an additional charge of 20 NOK on the price).