J.B.
My advice is kind of opposite of the other poster, but if at all possible (and I know it may not be) -start in the evening or late afternoon. Whenever we can, we do this and it gives only a few hours before your children sack out for the night. Often being in the car makes them sleep better and fall asleep earlier. I like to take the bulk of trips while they're naturally asleep. Any awake time -try to fill with favorite books and new ones and some new, inexpensive travel toys -and yes, dole them out one at a time. The little travel Magna Doodles, Play Doh and books with flaps and levers to push and pull have been successful for us. If you have a vehicle with DVD -use that or consider buying a portable and popping in a Disney or Pixar film!