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Well, there are several hints for traveling as a family here, but I think foremost are these two.
One: Get the kids involved. Sit down one (or several) night(s) with an atlas (for detail) and a wall map (for marking) of the US. Start marking the places that most or several people want to go. Pretty soon a pattern should develop whereby you can connect most (but sadly not all) of the dots, and that will be your route. If you assign each child a few of those spots to research, it will be up to them to find out what they can on the site and prepare a short (and age appropriate) summary of what everybody should be looking for so that you can make the most out of each visit. They can give their info in the form of a short talk as you are driving to the site or Mom and Dad can gather them all u beforehand and put them in a book which will also serve as a souvenir.
Second: Don't push it. Mom and Dad will be able to sit in a car for many more hours than the kids will. But to keep everybody happy, you should plan on stopping every few hours at smaller locations for a picnic lunch, short walk, or just to get out of the car and yell. If you break the trip up into several shorter drives with a worthwhile destination at the end of each small bit, the children will always be on a new adventure rather than just slogging down the Interstate hour after hour. And every so often, just take a day off and sit beside the motel pool.
Finally: National Parks are one of the best deals going. If you plan to visit 4 or more, buy a Golden Eagle Pass at the first one you come to. They're $80 and get you entrance (but not other fees) into all National Parks and Monuments for a year from purchase. And your children are all within the age range for the Junior Ranger program. Ask at each park. They'll get a set of activities that will help them get more out of the park, and a badge for each park where they successfully complete the tasks.
AZBuck