Depends a lot on where you are and how long you want to go away for.

queensland as there is so much to do and because of the weather in particular you aren’t limited

We recently went to Port Douglas and it was pure heaven,there is so much to do or you can just put your feet up and chill!

I like all of Australia and it is so hard to choose. Coastal areas are always good with children. The major cities of Australia are also worth a visit as well as some of the sleepy little towns. I would suggest getting onto NRMA and other sites to find child friendly places.