You could definitely do longer than a night or two in Tassie. A couple of nights in Hobart and some time in the national parks would be lovely. You can fly and rent a car or take the Spirit of Tasmania ferry over, I think it goes from Geelong these days.
Back in Victoria the Great Ocean Road is a must see. You could do that over a few days as well.
The Yarra Valley is the wine region of Victoria. There will probably be wineries that are kid friendly with lots of outdoor space and maybe even some animals. I’ve been to ones like that in Orange in NSW.
The Grampians are also beautiful, lots of bush walking and kangaroos. There are loads of places to stay.
Australia is all about nature and getting outside. The national parks are stunning so I would definitely recommend you visit a couple. Each state has a website for their national parks, so definitely check out Victoria and Tassie sites and all the info you need will be there.
A few things to consider:
In March it’ll be getting cooler down south, especially if you go inland away from the coast. Make sure you bring jumpers and even a coat each. I know how this sounds but check the distances when you’re planning a route. The roads aren’t as direct as they are in the UK and the service stations aren’t as frequent.
Melbourne is nice but unless you’re catching up with friends all the time, there isn’t three weeks worth of activities there, even with day trips. You should definitely look into a longer trip or a few shorter trips around Victoria and Tassie. How is your three year old in a car? As soon as you leave Melbourne you’ll need one.
Sorry if this seems patronising! I’ve been here 10 years and when I first came I had no clue about the distances and how much time is spent driving!
If your trip falls over Easter holidays book early as everything gets full very quickly.
I hope you have a wonderful time, it’s a beautiful part of the world.