If anyone clicked on this, as I did, thinking it might be about the overnight North Sea services from Harwich to Denmark, The Netherlands etc. operated by DFDS, then your experience may be a bit different to the lovely sounding French route!
Mini-hijack, but I couldn't bear anyone to go through what we did last Summer!
There is little, to no entertainment for young children and the public areas of the ferry are tiny and mostly focussed on getting you to buy expensive gifts and alcohol. You really will spend all your time in your cabin if you have kids, unless you're prepared to pay a fortune for a not very good dinner, or have them sit with you in the bar.
There are no lounge areas, other than some executive area you have to have a loyalty card for. The 'children's play area' is a small area (think the size of a small sitting room) off the cafeteria, with some lego (of course!) and a domestic size television showing cartoons. There is sometimes a man doing balloon animals, limbo, hula hopping and some 70s era 'humour' (mother in law jokes, that type of thing.)
Everything costs a fortune e.g 15 Danish Krone for a small bottle of water. (That's more than £1.50.) Take food, drinks and snacks with you and picnic on the floor of your cabin.
Allow for the possibility that if some, or all of you get sea sickness, you may not leave your cabin until you dock and that 12 hours is a long time with no drinking water! (Or food, for those who are well enough to face it!)
Breakfast is plentiful and mostly of a good standard but much, much cheaper if you book it advance. It's better to go early for breakfast as they stop replenishing the popular items towards the end of the service period. Of course, if you're too ill to eat any you've waved goodbye to the £15 odd quid per head that it costs!
Despite all this, I would do it again! Especially if someone could guarantee me a calm crossing.