J.J.
I got lots of info from http://www.nhtsa.dot.gov/people/injury/childps/WHALEBroch... Including the two links below.
http://www.nhtsa.dot.gov/people/injury/childps/WHALEBroch...
http://www.nhtsa.dot.gov/portal/site/nhtsa/template.MAXIM...
The last one tells you that the child needs to be atleast 8 years old or 4'9" tall before they can sit without a child safety seat and breaks down which seat is necessary for them. The first link gives you lots of links to look up information to keep your child and you safe in a car.
We take a trip to Arkansas every year with three children 13, 5, and 2 is there ages on our last trip. If you can afford one of those dual dvd players for the car that is seriously the best bet if you have more than one child. The reason for the dual is becuase they can watch the same movie from two different screans or they can watch two different movies with earphones if they like. Take all their favorite movies so they can enjoy watching them. We also stop a lot and let the children stretch their legs. Some of your rest areas do provide child play areas for the kids and letting them play for 45 minutes before putting them back in the car gets you to your stop a little latter but you will have less headaches as well. The dvd player really is a god send when your traveling. Even if you don't let your children watch a lot of tv at home giving them a treat like this on a long drive will make your life a whole lot easier. Toys and crayons fall and you can't always turn around and help your child find them but the movie stays where it is and is at the top, behind your chair and is easier to access to change a movie.