For some colour to your post, we are a young family (2 kids under 5) spending 3 weeks in Spain in October (also from Oz).
Our itinerary so far:

Barcelona (3 nights)
Malaga (2 nights) then bus to
Granada (3 nights) then hire car and drive
Cordoba (2 nights) then drive to
Seville (4-5 nights)
Barcelona (5 nights)

It's looking tough because of the kids really, but we are quite realistic that we'll probably only see 1 or 2 attractions in each location....and that's ok because if the kids crack it, it won't be worth it.
We also thought about dropping Granada but in the end i just couldn't bring us to do that given it's probably another decade before we'll go back to Spain.

Good luck with the planning and looking forward to your trip reports!