If your parents were citizens of Sweden, Denmark, Finland or Iceland, and became Norwegian citizens by submitting notification before you turned 18, you automatically became a Norwegian citizen if you

Children under the age of 18 automatically become Norwegian at the same time as you if your child is resident in Norway and is not married or a registered partner (see the Citizenship Act § 22).
Before you can apply for a Norwegian passport for your children, they must be registered as Norwegian in the National Register. You must contact the Tax Office yourself to do this.