Hi cheekychops03,
We've just returned from a trip of a lifetime to Australia - initially we were like you and wanted to take on the world, however realised it wasn't realistic.
Our dd was 22 months when we left and he relished every moment being with his two parents for the first time ever 24/7. And we've all grown so much as a result.
Australia was amazing. Not the most child friendly though but then you may find it easier with a 4 year old - for example, many restaurants in most cities did not have high chairs or a baby change facility in their bathrooms! But then this should not worry you.
The people in Australia are very, very child-friendly though, so it's a great country to explore.
We travelled by air but would next time consider driving the Great Ocean Road (too scared to this time for fear of making our son too bored!!) Sydney was fantastic - stayed in an apartment in Manly with the Pacific crashing on the beach sending us all to sleep at night.
The other thing to bear in mind I guess is cost - and since we've returned to the UK, we cannot get past how expensive EVERYTHING is from nappies and normal groceries to your casual coffee and cake in a Starbucks!
Eating out is very cheap - everything you see in Australian dollars would be the same in Sterling e.g a coffee would be $2.50 whereas we pay £2.50 - so in Australia, you pay just one pound for something which we pay £2.50 for.
So yes, your Sterling will go far in Oz! Something to keep in mind.
We'd do nothing different expect stay longer and take a little folding / cloth seat to strap our dd in for when we ate out.
Enjoy your trip. My advice...go for it.
Already planning the next trip...India or South America...who knows!!!