I don't know either, and would love to find out where 'Watson' came from. She appears to have changed it somewhere between 1867 and 1872, and I suspect it had something to do with her parents continuing life of crime. I think her father was sentenced to 14 years hard labour in 1866, and he is mentioned in the court notices of Adelaide newspapers from 1850.
She was taken in to care at the age of 9. I don't know if this was temporary or permanent, but if it was permanent she may have been fostered and taken a foster parents name. Not sure why she would have waited so long though.
Her mother had a couple of aliases, but not Watson as far as I am aware.
Also, I only have records of 12 children (hopefully I will get them on to the family tree soon). John (1862), George (1863), Arthur (1865), Elizabeth (1867), Mary (1870), Alice (1871), William (1872), Harriet (1876), Robert (1877), Rebecca (1879), Adelaide (1881) and Albert (1884). Can you tell me who I am missing, and how you found them? John and Elizabeth don't seem to have been particularly diligent about birth registrations.