h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

One way is to cover there mouths with duct tape and put them in a trunk, but they do not like that (I should know, I once was a kid). One thing that works is to let them stay up the night before the long journy, and leave very early and tell them to sleap on the way to the place you are going. My parents did that to me. It also makes them think the got there faster because they were K.O.ed.
The last thing that I can think of is to bring a coffee cup, a letter opener, a family pet, and a lite bulb into the back seat and let what happens happen.

-Sean

P.s. I just remembered, game boys (Oh wait Game Person(to be politicaly correct)), laptops, or any other thingys that can use batterys or can be pluged into a ciggeret lighter will also occupie them.

Another useful method of keeping kids quiet for long trips which does NOT include the use of valium is to invest in a portable DVD player.

DVD players are small relitive inexpencive devices which play DVD movies, a DVD is a disk with a popular movie burned onto it in much the same way music is burned onto a Compact Disk. With the correct movies one could keep the little buggers spellbound for hours examples of long hall DVD movies include the 'Shrek' series and the ever popular 'Harry Potter' series.

If you do not want to get a DVD player for your motercar then you could always play 'find the house elf' a Harry potter theamed game which will have the buggers glued to the windows scanning for house elves for hours