IIRC, with Eurotunnel, you'll basically be in the car, though there is the opportunity to get out. It's not super interesting, though, as there's nowhere to go!
Have you got room for an adult in the back with them. I would definitely do this for at least some of the time, as even drawing is hard in a carseat when you're little and they can't reach stuff they've dropped on the floor.
Doctor's kit is great, as is some toy tools. You can fix everything in the car. Playdough is definitely worth the possible mess associated with it. As are all the other suggestions made by the previous posters.
Magazines, some for reading some for shredding (we bought DS a bus enthusiast's mag which he chose himself and he adores it. He read it for three hours on and off of a 7 hour train journey)
Singalong - all occupants of car must join in. Bring favourite tapes/CDs. Make up new actions to well loved songs.
Those little icing pens for decorating biscuits. (and squeezing direct into the mouth) Saw this done on a train and it was fab.
Time as much journey as you can at sleep time and let them sleep. If it were me, I'd probably spend the afternoon mooching about Calais with kiddos and then head off on the long leg of the journey straight after tea, so you'd only have 2 hours to go before they slept. But it depends on what other constraints you may have.