You will be fine and you will have a good and easy time!
When you get on the boat, go and find your cabin. It can be quite a melee at times, so it really does pay to get to Portsmouth early if you can. Otherwise, just let everyone else rush around for 15 minutes or so, and then find your cabin. All the beds will already be made for you. If you have booked a 4 berth cabin you may find that the two top bunks are folded up to the wall, so that you can use the cabin as a sitting room - but they are easily pulled down from the wall.
There is a little shower/toilet room ensuite, which is tiny, but there are small towels and soap.
You will probably find that in the morning you don't want breakfast as it will be pretty early when you dock. What we usually do is get off the ferry and then go and find somewhere in St Malo for breakfast. Parking in St Malo isn't that bad, as long as you don't try and go into the city walls.
On board the ship there are (usually) two restaurants - one is more like a self-service canteen, and the other is a proper restaurant with waiter service. The food in both isn't bad - up to you if you feel your DC are up to a proper restaurant! The canteen/self-service usually does some kind of happy meal thing for children - if yours are still into things like that!
I am very envious of you. I always looked forward to the overnight crossing - felt like the real start of our holiday.