lost family

Ive managed to research quite a lot about my Norwegian ancestors but there is one thing that bothers me, my gr grandfather, in the 1865 census had 8 children yet the only one i can find is my own grandfather who came to Scotland in 1887, ive looked up the 1875 and 1900 census and only in the 1900 census do i find my great gandparents, they were originally from Fredrikshald(Haldane)but by the looks of it spent most of there life in Onso, the children are Aksel ,.Karsten, Kaurin, Harold, Olaf born about 1857, Petronnelle Valborg, Hansine all have the second name of Pederson, was there some kind of tragedy in Norway between 1865 and 1875, that maybe have caused them to disappear, can anyone help