Hi Felina,
How many days in total are you planning for your Australian trip?
I notice from your post on the Australian forum, that your original plan was to visit Melbourne, the Great Ocean Road and Phillip Island, renting a mini-van. Agree with the advice that the GOR would not be ideal with 2 small children and can understand your decision to drop that.
The other parts of that Itinerary - Philip Island, Melbourne and the Grampians or Ballarat would certainly work.
So would Sydney, the Blue Mountains another out-of-town trip. Two elements of your proposal won't, though:
1) A mini-van in Sydney city is an expensive, unnecessary nightmare. Don't rent it until you are ready to go to the Blue Mountains or elsewhere out-of-town. Sydney has excellent, cheap public transport.
2) Accommodation in Mascot has nothing to recommend it for visitors and less so for those on a short Itinerary like yours. Whilst Sydney has great public transport, you don't want to spend too much time & $ getting from your accommodation to the places you want to visit.
Here's an interactive map of Sydney that will be useful in orientating yourself.
Sydney map:
https://goo.gl/maps/PFmeA45BAKXruh4F8
Use http://www.booking.com.au to find more centrally located accommodation for your family. We'll be happy to help you narrow down your choices if you give us a nightly budget in AUD.
This is a good reference for things to do & see in Sydney.
Visit Sydney: https://www.sydney.com/
Sydney is located on a justifiably world famous harbour. Trips on harbour ferries to Manly, Watsons Bay and any of the inner-harbour suburbs are highlights in themselves as well as transport links to interesting places with lots to see & do for all ages.