Another newspaper record relating to his death says "five children"
http://trove.nla.gov.au/ndp/del/article/39552115There are some images of Tasmanian BMDs now on familysearch.
Here hopefully links to Ethel Maud:
http://www.rootschat.com/links/0u5j/ (if not it's under "Launceston and County Districts, RGD 33/64. Births, 1885, Launceston and country districts, B-Victoria, page 290")
Only additional information - birthdate 6 August 1885, registrar informed by letter from Elizabeth from Weldborough.
There are indexes - but only the three Ah Kin children you already have seem to be on them (including son James as "James Tas Ah Kim"). Grace Lillian's birth is here also:
http://www.rootschat.com/links/0u5n/ It seems likely that there were children born between the marriage in 1875 and the registered birth of Ethel in 1885 (might depend on how they encouraged/forced registration). If George was an informant at his mother's death I'd imagine he was born late 1870s.
There was a "George Kin", a Chinese interpreter, in the early 1900s in Tasmania who seems like a good possibility for "George Ah Kin" the son of Elizabeth.
Court case reference (might indicate that he was not born China although not entirely conclusive)
http://trove.nla.gov.au/ndp/del/article/50661648?searchTerm=%22George%20Kin%22Death in 1919 for George Henry Kin, aged 38, husband of Ruby Myrtle:
http://trove.nla.gov.au/ndp/del/article/51051778His will is on the Tasmania archives and confirms that the man who d. 1919 (probate 1920) was a Chinese interpreter.
His wife may have been previously married to John David Chapman - there is a Trove reference for this man's death in 1908 (and the sad death of a child to John and Ruby in 1906) and there is another will in 1983 for a George Albert Kin who was unmarried but had a niece and nephew named - one a Chapman.
I think Ruby must have remarried as George Albert Kin has a WWII record with next of kin as "Ruby White".