Frithjof Johansen and Julie Iversen

I am trying to locate what became of: Fritjof Johansen b. 16 June 1859 in Kristiana married to Julie Iversen 3 May 1884 in Oslo their children: Ivor b. 1884 d. 1885 Borghild b. 1886 Aagot b. 1887 Astrid b. 1889 I can find no record of any of them after the 1900 census in Kristiana - where should I look to find out what happened to them? Thank you for any help you can give me in pointing me in the right direction!