I've done that route quite a few times. Would much rather go by ferry than plane on holiday!
The Portsmouth Caen route overnight doesn't give you a huge amount of time to sleep. Definitely book a cabin to give you all some time to sleep and a bit of privacy. They'll do travel costs for under 1s.
Worth getting the children changed into their pyjamas whilst waiting in the ferry queue.
They will park you very close up to other cars on board, you will not have much space to root around your car to find anything or even open the door properly to get out. Get a grab bag(s) with things you want for overnight. They'll give you a little card to say where you're parked.
IMHO even for the daytime journey its worth paying the not very much to get a cabin (they're much more overnight). It gives you a base to dump stuff. Fine to take food on board; but the food on board is reasonable and tasty (it satisfies the French ;))
Portsmouth harbour has lots of things to see and it's well worth getting on deck as you sail back in.
Do you have a long drive either side? Although we live close by, we tend to have a pre sailing meal at gunwharf quays and watch our ferry sail in to 'start' our holiday... Close to Caen are a lot of the D Day beaches and memorials, and you can stretch your legs and have some fresh air if you've had a long drive the other side.