Whereas you would need a couple of lifetimes to 'see it all', I still think that 3 months is a good amount of time to visit and is something I [and many others] can only dream about, so a "Short time frame" as mentioned by Lifey is relative in more ways than one.

I also agree that paper maps are essential when planning and while travelling, they give you a better overall picture of the terrain and attractions etc. However you will find on line mapping tools a big help in your planning and such a thing can be found in the 'Maps' link above. The Map centre lets you customise routes and find attractions and will help you to figure out distances.

As I mentioned earlier, it is up to you how much you want to do and how you divide your time. Putting it into monthly sections as suggested above will help you to break it down and perhaps seem less daunting. For instance if you want to spend several days or a week in any one place [New York for example] that will take a chunk of your time from that part of your journey. If on the other hand you want to spend a couple of days 'here' and 'there', but keep on the move [say in Colorado/Utah] you can cover more ground. These are the type of individual choices that only you and your family can make based on your style of travel. For example, a 10,000 mile trip around the country over 90 days is a little over 110 miles per day, 2 to 3 hours of travel every day depending on the roads you are travelling, or for each 3 days you spend in one place, you would follow up with a full [but comfortable] day on the road.

That's the beauty of the road trip, you get to choose, to create your own dream trip, and the planning is a big part of that fun/dream. Get the Kid's involved in the planning so they have a vested interest in the trip, if they feel part of it, the whole family will have a lot more enjoyment.

Keep asking questions as you go and enjoy the planning !