We did 16+ hours last year, with 4 kids, and they traveled great.
We leave VERY early in the morning so they sleep the first few hours. We plop them in the van in their pjs and change them into clothes before going in to eat breakfast.
Each kid had their own plastic box with age appropriate toys. The older ones got a new Nintendo DS or PSP game and/or CDs, handheld games; the little ones smaller toys, crayons, coloring books, Color Wonder markers and paper (FABULOUS!) and everyone had an inflatable travel pillow which they all loved because they could nap
We also packed a box up front of family items: travel size games (Battleship, Connect 4 etc) and we found great travel games in the dollar section at Target: license plate bingo, travel bingo where you looked for certain items on the road ex yellow car, bullodozer, no passing sign, icy when wet signs etc. If anyone got antsy with their stuff, we could play a family game. We bought a few new movies for the DVD player and packed lots of snacks, juice boxes and bottled water.
We also took balls to play with when we stopped for gas or to eat and would let the kids run for 5-10 minutes before getting back in the van.