That's the Newell highway.
We stopped in Dubbo overnight and did the drive in two days stopping every three hours or so.
I think you do it it in three days driving 8 hours a day.
It takes about 17 hours of straight driving to get from Melb to Toowoomba so add a couple more hours to get to the coast.
The drive is pretty easy.
We stopped in Shep from memory, Dubbo (overnight), Moree and then drove all the way to Toowoomba. They were the major stops were some smaller pit stops along the way at parks/servos.
DD was 3 at the time, only had the one then, she was well over it by the last couple hours but we did the drive in two days.

I'd also go overnight on the boat. It's a long time to keep the kids occupied and there just isnt that much to do on the boat after a while. If its raining then you have to stay inside, too hot and you'll all get burnt from sun wind water reflection. So while it is doable, I think it would be much easier at night. Just depends on how game you are

p.s just re read what I wrote and its def time for bed, can hardly write a sentence so I apologise lol