We have been on it last summer - 4 adults + 1 x 11 month old.
There is a LOT of hannging around at both ends which was the most annoying thing for us as the terminals do not have a lot of seats/facilities. They are literally little sheds.
I'm not sure about the sharing a cabin thing - I don't think you have to anymore - you get one cabin per car.
It is noisy and pretty warm on board.
I think it's pretty well organised.
There is only one dining car but we just took all our food and drink on board with us.
The gap between the bunk beds is pretty narrow so our 11 month old slept in his carseat propped betweeen the bottom bunks which meant it was pretty well reclined and also fairly comfortable for him. Your 3 year old could sleep on one of the bottom bunks no problem.
The bunks you don't use fold back against the wall so you get a bit of extra breathing space i.e. not all 6 have to stay pulled down.
It is SUPER expensive for the accommodation and I have heard things about cars being broken into etc. But, on balance, for us and the amount of stuff we wanted to take it definitely worked out pretty well.
I'm not sure I'd definitely take it again but I'm not sure I definitely wouldn't IYSWIM.
I think it's worth a try......it's only two nights out of your life after all!
G