This tradition was introduced by King Haakon in 1906, and today 17 May and the children's parade are virtually synonymous.
Norway's national day is not celebrated with military parades, but with local children's parades made up of school children marching behind an amateur brass band. In many places, there are games and other fun activities for the children after the parade. It is a festive day for everyone, but perhaps most of all for children. They are dressed in their best party clothes, and can enjoy lots of hot-dogs, ice cream, fizzy drinks and cake.
Also outside of Norway, Norwegians celebrate the constitution day. In the Netherlands, the 17 May is usually celebrated at the Norwegian church in Rotterdam.