Archibald's Will is available on familysearch.org. It names 2 daughters and 4 sons in his will. Paperspast NZ can be searched with many results for Archibald and family. He appears to have been married twice. First to Eliza Manning Wright Palmer in 1854. BDMNZ records them as having 3 children.
1855 - Selina. Selina died 5years old 30 Jan 1861
1857 - Eliza
1860 - Louisa. Louisa died 11mth old 13 Feb 1861

Archibald's wife Eliza also died in 1861 - on January 11. Death notices available on Paperspast NZ. There must have been some illness about. Possibly Scarlet Fever but you would need death certs to confirm this.

Archibald remarried in 1866 to Louisa Ann Stone. They have 5 recorded children on BDMNZ.
1867 - Archibald James
1870 - William
1873 - George Arthur
1875 - Edwin Charles
1878 - Sarah Alice

Archibald's children, Eliza, Archibald, William, George, Edwin and Sarah are all mentioned in his will.

Archibald Obit in The Thames Advertiser https://paperspast.natlib.govt.nz/newspapers/THA18950327.2.9?end_date=31-12-1895&items_per_page=10&query=archibald+somerville&snippet=true&start_date=01-01-1895

Hi again Den,
Archibald actually had 8 children in total from 2 wives
You will find them at this FAG site. There you will see his two wives and 6 of his 8 his children in an easy to see list format.

There are still 1 child missing from the list ..
1870 - William Somerville as I have not yet found his death date

I have also submitted additional info for your grandfather Edwin so go back from time to time

Thank you both for that help. I am working through adding this to my Family Tree information.
Den