Thanks for the well-wishes and ideas. The kids did great on the car rides, even the day we spent over 13 hours on the road. I think that it helped that I only made a minimal effort to entertain them, so they mainly entertained themselves, rather than looking to me for something new to distract them every 5 minutes. That's the big advantage car rides have over aeroplane rides: you don't feel like you have to keep the kids completely silent.
The magna-doodles were the most successful. Usually I take the small 4 by 6 inch ones we have on trips, but this time I took the larger ones and they kept the kids occupied for far longer. They also enjoyed the 'new' toys, such as their felt boards: One piece of stiff felt (instead of the usual sandpaper board), plus a bunch of shapes cut out of felt - random shapes for my son and more specific shapes for my younger daughter - to make pictures with. The metal boards and magnets (that I had made out of magazine pictures and those sticky-backed magnet sheets) were preferred to sheets of stickers. And for the moments when bribery for peace was needed, snacks and a couple of small surprise gifts did the trick... Phew.
nb. We didn't get a dvd player. Not because I have anything against the idea of them, but because every single one seemed to have at least one customer review saying that it had broken down on the first use. I didn't have much confidence in our luck with electronics...
I had better go and do some laundry. Take care all.