Are these the children of William August Edvard Hartman and Emma Nicoline Holm that you have?:
All in Helsingor Parish, Fredriksborg county, Denmark
1. Gotfred William Arnholt Hartman, christened: 20 May 1872 in St. Olia Church
2. Emilius Carl Frederik Hartman, christened: 21 Mar 1875 in St. Maria Church
3. Emma Emilie Hartman, christened: 29 Jan. 1874 in St. Olai
4. Anders Johanes Hartman, christened: 21 Sept 1878 died: 19 oct 1878 in St. Olai
5. William August Edvard Hartman, christened: 11 Jun 1880 in St. Maria
6. Emma Nicoline Hartman, christened: 7 sep 1884 in St. Marie Church
7. Niels Holger Hartman, born about 1877/8. (on the 1880 and 1890 census of Helsingor, Fredriksborg, Denmark.)
[Children 1-6 were found on the IGI or VRI on FamilySearch.org for extracted parish records information, they were on films # 300694 or 300693]
Also the marriage record was there:
William August Edvard Hartman, age 27 (born 1842)
Emma Nicoline Holm, age 23
married 18 Sep. 1869 in St. Olia church, Helsingor, Fredriksborg county. (film # 48577, marriages in Helsingor)
Your orginal question was finding William August Edvard, born 1880 and his sister Emma Nicoline born 1884. They are not on the 1890 census with their father and brother Niels Holger. Could they have died by this time? I looked all through the parish of Helsingor and couldn't find them. Then I looked all through the district of Lynge-Kronborg, where Helsingor is, and I couldn't find them.
Perhaps you could do another query just looking for their death dates on Arkivalieronline.