This is another online database that allows you to search for births, deaths or marriages in Western Australia. For this search I haven't been able to locate a suitable 'candidate' with Shetland roots. However, I have been able to provide a good example searching for children born to Alexander Ross.
This is the search form where you are first required to check the radio button against either Births, Deaths or Marriages.
For this search I have used 'Ross' for the Family Name, 'Alexander' for the name of the Father and entered the period 1900 to 1932. This should provide the names of children born, registered and in the database.
The image above shows the results of the search. You will notice that the results are given for different names for the Mother. Here you then have to make a choice and see how it fits. If you already know the Marriage details, then you will know the mother's name.