Money
Agree with all previous posters - as a tourist just ensure you have WeChat and/or AliPay set up before you go and you will be fine.
Note that some functions of WeChat (such as receiving and storing RMB on your wallet) will not work overseas unless you verify with a mainland bank account, but magically works fine once you are logged in from mainland (e.g. from the free airport WiFi).
Unless you are travelling out to the sticks where there is limited internet, chances are you will not use any cash unless you specifically try to do so at larger establishments where the typical spend is in the hundreds.
e.g. In Shanghai, I had to spend my cash (which I received from relatives) at the hotel as most smaller shops are completely cashless so wouldn't have had any change for $100 bills. The poor receptionist who was in training had to "go to the back" and fetch a bundle of coins and small bills for my change, which I then promptly stuffed back into the charity box once she dutifully counted it all out.
Internet
eSims are the way to go, leaving your Aussie physical sim on roaming (no data) so you can still receive SMS and do WiFi-calling (if using Telstra/Boost). Really easy to get cheap deals from Klook, Simify, TravelKon, etc. It'll be pretty much impossible getting a physical mainland sim unless you have local ID card, or otherwise risk buying a dodgy one (usually registered to someone else and resold).
Unless you are truly never using streaming/social media, I'd say you should buy at least 2GB per day since you will not be using hotel/public WiFi much at all due to GFW restrictions. Pretty much all Google services are blocked as is most Microsoft cloud-based services and Facebook/Twitter. For a typical Aussie, you will be forced to live a totally alternate reality if you had to rely on local versions of these services!
Transportation
Didi is your friend in all urban areas, quick and simple just like Uber in Australia. You can access Didi directly from within Alipay or WeChat apps.
While public transport is perfectly fine in big cities (and cheap), you lose out on convenience which means less time for activities. A one hour trip by public transport, especially if there is walking involved at both ends, would typically only take 30-40 minutes by taxi from door to door. The time saving and convenience factor far outweighs the cost if you are travelling as a family - not fun dragging small kids through stations at rush hour!
Also if you are travelling with kids, most city public transport services are free for children up to 120cm. Just tap on for yourself and your kids tag along without needing to pay. If you have older kids, they will likely have their own mobile phones so can just download Alipay/WeChat on their phones (which you can delete after the trip if needed).