Temperature will be between 5C (in the morning and at night) and 12C (during the day) in Melbourne.
There will be quite a lot of things for a 3yo kid. And the following places are suugested:
Melbourne Zoo www.zoo.org.au/MelbourneZoo
Melbourne Aquarium melbourneaquarium.com.au
Melbourne Museum museumvictoria.com.au/melbournemuseum
Queen Victoria Market www.qvm.com.au
Philip Island (to see penguins, chocloate factory, kangaroo farm etc) visitphillipisland.com
Enchanted Maze www.enchantedmaze.com.au
Go to Mount Buller , join day tour from those Asian travel agent , pretty good and cheap. But you need to rent gear to get up the mountain, normal the tour guide will drop off in the morning in Mansfield for you to rent suitable equipment.
As if you have a 3 year old , sight seeing tour will be enough and i think about AUD $68-$99 each adult.