We've just done a similar journey with 3yo, 2yo and 7month and found a car DVD player is probably the best entertianment. Books/magazines, colouring books, small toys are good too. Playing I spy, singing along to cd or radio.
The only car friendly snacks we've found are small bags of fruit as even if some of the fruit gets dropped it's easy to pick up as it doesn't leave crumbs.
Leaving early in the morning helps too as the kids slept a lot of the journey and letting them run around at services even for five minutes got rid of some of their restlessness.
Food : Raisins are also good - my local poundland are selling these little packets of annabel karmel biscuits which my two devour.
Toy wise : the mini aquadraws are great as its just water so no crayon or pen stains. Toy cars, finger puppets, picture books etc We also play I spy and who can spot the next red car or the next bridge etc
As passenger I am the keeper of snacks and toys and dole them out when needed then when they've all been dropped on the floor we stop and pick them up and start again