No idea on the best way to get the family to Victoria. I just take my bike on the plane with me.
Maybe box up the bikes and trailer and use Greyhound freight and pray that you bike is at the depot when you arrive.
I did the ride a number of years ago from Phillip Island to Buchan. There was a couple of big hill days on that ride.
The route profile here
looks like day 4 will be fun. 25km at 2.5% to 3% . I am tempted to do the ride.
I hope your training hard. There will be a few hard days with a trailer. I can think of nothing in WA like them.