Been over and back on the boat with a 1yo and an almost 4yo.

You are doing the right thing by having a cabin at night (that was the first thing I was going to suggest when I read your thread title). I have had a couple of friends with kids who had day bookings and changed to night for the return trip.

Really it is not much drama, you go to sleep, wake up and drive off the boat. The biggest pain is the wait in line, thankfully there are toilets in the waiting areas. Just don't leave the car for too long, we almost lynched a guy who walked off for over an hour, so our lane could not move when needed.

Food is expensive on the boat, so if you do want to eat at the buffet. Just have the adults grab a plate and pile it up to share with the kids. For brekky, we just found the nearest Macca's in Devonport once we got off.

As for seasickness, maybe take some tablets before hand. I was taking ginger tablets. Due to the time of night, I just went to bed with the 1yo and the boat rocking put her to sleep. We found that we shared the bottom bunks with the kids, even though we had ordered a cot.