I think the kids would handle it fine, for me it would be more about how you will be the next day with minimal sleep for both you and DH and then doing another drive. When you get to WoyWoy are you going to try and rest?
For us we drive during the day as the my eldest is just under 5. We drive a few hours, stop an hour, drive a few stop for one and so on or until they can not handle it anymore. When they are older I would love to change to night travel but DH is not so good at night driving, he just gets too sleepy. So I would have to do all the driving and I can not sleep in a car either.

Anyway enough about me.
You can get kids neck pillows that might help with the head falling forward.
That is a valid point. In the past I have been able to sleep well in the car, on planes, trains, buses etc, so hoping that once the kids are asleep, I will sleep too.. We used to do a lot of night driving, well DH drive and I sleep as he has been a night owl and worked shifts most of his adult life.

DH will be able to sleep at my Nan's and I was going to take her and the kids out for a few hrs so he has some quiet for that reason, same as at home, we go out while he sleeps so he should be fine to drive the rest of the trip that afternoon (it is about 90 mins to where we are going).. My nan actually lives where I grew up, so there are a few things I would be able to show the kids through the morning.

We very rarely make rigid plans, and the only thing that is a must for this trip is that we are in Sydney by the Sunday evening, so we can get sorted to get to the cruise terminal the next day (leaving our car at families home).. So deciding we have to stay somewhere to sleep/rest isn't really going to make too much difference, and will be crossed as we come to it..