Thanks all for your helpful replies!
It seems J&E has actioned the course I am contemplating of applying for a 600 while awaiting a 300 decision.
If 300 not granted when I return to TL in December and submit the passport application for our son with his newly acquired Citizenship certificate
My only concern would be Westley's comment
" As a rule delegates let applicants in Australia know that they must leave Australia long enough for the visa to be granted."
Do you know of circumstances in which the applicant/sponsor is not advised to leave the country and the 300 decision is suspended?