I always pack loads of nibbly food that takes ages to eat. I try to limit sugar for obvious reasons.
We have a fab travel Bingo game in a box in the shape of a car, think it's French. We give them a sheet at the start and they have to cross of things as they see them. We got it from Cox and Cox I think. You could make your own great for kids that chunder as they look out the window.
A roll of foil is good to make models with.
We have those neck rests made by the Trunki people in the shape of aliens. Ours play with them too.
Travel pockets are a must,each kid has their own tied to the seat in front so you don't loose anything.
Top Trumps are good but maybe not for the chunder kids.
Cd story tapes only to be used in traffic jams are good-Horrid Henry saved our life but you have to live with the aftermath when you get there ie The Bottom Song,re-naming teddies Mr Kill and calling siblings 'worm'.
Travel etch a sketch things from ELC are good-who can write an a,draw an apple,write boat etc?
For babies a bin bag or toys you lob over your shoulder is good.Also Evian sprays if you have no air con.Boxes of raisins.
We have avoided car DVD players as I think it makes kids lazy at amusing themselves, I also think it's a shame that they miss the different scenery. Having said that I would have happily given my right arm for one last year on the way down. Having stuck it out though I noticed ours were heaps better on the way back as they were used to the boredom iykwim. Also everybody has different tolerance levels,I'm used to mayhem on journeys and just resign myself to it. I know it can get people down though.
Always over estimate the time you need,keep on the toll roads as much as you can and find hotels just off the main roads. We've made the mistake of booking lovely hotels that look near to our route on the map but when you get off we've literally lost hours in time going down endless country roads,not good when you're at the end of a journey.
Go for hotels with a restaurant,you don't want to be food hunting on arrival.
Eat breakfast as early as you can and get on the road you're always glad of the time gained more useful to use the extra time for wee stops than a lie in.
Plan for travelling on Sundays as shops off the beaten track are often closed. Nothing worse than turning up to a supermarket with an empty car park and tumble weed when it's 12 o clock and you're all starving. Think things are better these days but asume any supermarket off the main road could be closed,if you see one open grab it. Lunchtimes on any day are hit and miss too and were/are Mondays??? Could be wrong re that one.I pack a jar of peanut butter and rolls just incase we can't get bread. I always have bag fulls of snacks,water,oatcakes,fruit somewhere in the car so if we can't get nice bread and cheese we don't starve.
Fill up with petrol last thing Saturday as pumps will often be unmanned on Sunday and British cards don't always work although this is getting better.
If your French isn't great have a dictionary in the car for petrol pump reading emergencies.
Avoid motorway services in August always heaving with queues,overflowing loos(holes in the ground) and general mayhem. If you see anywhere nice to picnic with a climbing frame grab it as the kids get a run around which really helps. Start looking for lunch stops at 11.30.
If you have a chunder kid pack loads of travel sickness pills,triple what you think you'll need. France is so much bigger so you will run out. I only like giving dtwin1 the pills with the ingredient I know he tolerates well,not sure if it's as available in France. Also nothing worse than chunder in your car,you'll be living with the smell the whole holiday which then makes chunder more likely for the other kids.
Hope this helps have more but starting to bore myself now