There is a death notice for Beatrice May WALKER (formerly THOMAS)
https://trove.nla.gov.au/newspaper/article/43497266 It names her children. In my experience, on these occasions children are usually named in birth order, (there are exceptions to most things

) but I do not see a daughter as you mention, listed as the sixth child. There may be all sorts of reasons for this. But...
I do see a male as number six.
There is tree online at Ancestry without much documentation which lists a man of the right (or nearly right )name as the son of Beatrice May GULLY and Charles Thomas WALKER. The name DAVIS has been inserted to read father Charles Thomas DAVIS-WALKER. The child is also named ***** Davis WALKER.
The tree says he was born in 1922 and a marriage is also listed for him.
It also says he died around 1997.
Does any of this make sense to you?
I mention it because the birth seems to post date the SA Police Gazette notice placed in 1921.
Sue