Distractions should also include colouring pages and crayons (so no sharpening of pencils needed en route, and no felt tips in a confined space!!) - print off a few free colouring sheets of their favourite characters (lots on web), and maybe a few activity ones if they would be interested/able for them. Have some blank sheets too for just drawing. And do a few (homemade/web) car-bingo type sheets - pictures of things like trucks, buses, windmills, factory chimneys, farm animals, different coloured cars, birds etc - to mark off.
Absolutely YYY to DS and travel DVD!!!
Plenty of snacks (preferably non messy) - little tubs of raisins and other dried fruit, rice cakes (chocolate or yoghurt covered ones maybe?), jelly sweets, tubs of cereal...And drinks in non-spill cups. Which can be doled out from the front.
Perhaps a few small things to distract after a while - like old or new toys wrapped up (worked great to have a few poundshop things wrapped on 6 hour flight last year for DD), a small etch-a-sketch/magnadoodle (just notebook sized) was non messy, little puzzle, new cheapy doll, car, etc. Dole them out sparingly and one at a time. Perhaps a new comic ("Friends"?)or a few favourite picture books if they are ok reading while travelling (some can, others can't). Or something to keep their hands occupied but not too small pieces - maybe a couple of cars or big duplo but not lego.
Have plenty of car-games in your head/on a list. Singing "wheels on the bus" or "Old McDonald" (I had a wide range of animals, including zoo animals, for regular commutes home in rushhour), counting different coloured cars, ..will try to think of more. You'll find lots of ideas on the web generally though (about.com can be great for things like this).
Have a spare change for all in car, wipes and nappy sacks (even if neither are still in them - or else bin liners - just in case you need to contain anything), tissues, travel potty if needed. Make sure the kids seats are comfy - we often put DD sitting on a cushion in hers (she tends to bring her cushion to sleep on anyway) for long journeys, we have a small rug that she can snuggle under if she's tired and might drop off, and she always has her favourite teddy too for a snuggle. Know how to put their car seats into a comfortable recline for either early start before they're awake, or for after a rest stop and hope they sleep then.