Making good time is NOT important with small children. Both of my kids have always been wonderful travellers and it's because we are constantly coming up with things for them to do. Last road trip (six days on the road there and back) included the flip trays (you can get Hot Wheels, Barbie, Scooby Doo ones - we got our's at Walmart) fully stocked with a cheap calculator (don't know why these are so fascinating to kids but they are), several favourite colouring books, drawing pads, pencils, washable crayons, washable markers, funky erasers (little cartoon character shaped ones). Handy wipes. Cheap cd player with headphones and LOTS of batteries. Children's music cds and the storybook/cd read-alongs are great and will be well used. Handy wipes. A cooler stocked with fruit and water bottles and juice boxes (there will be more than enough opportunities for junkfood). Handy wipes. Sunglasses and those pull-down, suction-cupped to the window sunshades. Handy wipes. A light blanket and a thicker blanket and a comfy, small pillow. And did I mention handy wipes? And a first aid kit with ANY medication you might need for your child - tylenol, ibuprofen, benadryl, cold & cough, gravol, etc.
We also planned at least one daily stop close to the highway we were on for educational purposes, but more so for stretching and moving around and preventing the kids from getting bored. One stop was a bird sanctuary (the kids LOVED it), one was one of those kid's themed play restaurants.
I would certainly advise against medicating a child just to make them drowsy - some kids become agitated with benadryl instead of sleepy. I'm sure that post was meant in jest, but you just never know, right?