Is it the Harwich-Esbjerg ferry you're doing?
We go on it twice a year to visit my parents etc in England, have been doing so for the past couple of years now, DS is now 3. Next time we go (December) we'll have a 3 month old as well.
We have one of the dvd players that go on the back of the headrests in the car, which is fab, certainly saves our sanity in the car, and also on the ferry as it also plugs in (they use danish plugs on board so remember your adaptors for anything electrical), so DS can watch his dvds there as well.
We always pay for the tv in the cabin, the films aren't bad at all, they have ones for adults and kids (and have had Happy Feet every single time we went!)
We don't bother with the food on board, dinner is expensive and we found it quite stressful, it is nice food though. We take a picnic on board so we can eat in the cabin. If you do eat on board you need to book, don't forget they use danish time on board so all tables bookings will be in that.
We also don't have breakfast any more, we take a multipack of cereal,some long life milk and some juice (and plastic bowls and cups, and cutlery) as we found breakfast expensive for what it was, and I suffer from sea sickness so it's always a bit hit and miss as to whether I'll be able to go down for breakfast at all!
DS gets his own suitcase when we go which he can fill with whatever toys he likes, there's quite a bit of room in the cabins for them to play on the floor.
They do offer travel cots, but we've never used them as we co slept anyway- bit of a squish as they're only single beds but it's only for 1 night.
They also do a kids club sort of thing, there's a treasure hunt, which your 5YO will definitely be able to do, it's in English and Danish.
We always enjoy our trips over, the staff are fantastic and can't do enough to help you, and everyone on board can speak English and Danish so never a problem getting someone to help you if you need it.
Sorry this is so long, hope some of it was helpful! Hope you enjoy your trip