I've done 5 long road trips with my boy (now 10), and I'm about to embark in anytime one this summer. I go from here to home (Texas) every time, but I take a different route each time, just to keep it fresh and interesting. It doesn't matter which direction you go, because you can find good stuff anywhere.
I'll give some general tips that can help with the concept:
Get a couple of audio books to listen to for the long interstate stretches.
Don't fixate on distance per day; find the things worth seeing along the way and plan the trip around them.
Allow time for diversions.
KOAs are great. The cabins are cheaper than hotels. They're family-friendly, they usually have pools, bikes, trails and various things to do. After long periods in the car, it's nice to get some activity in.
Find the local flavor. Look up restaurants, museums, and attractions that are unique to the area you're staying in. I've gotten some great ideas by posting in the HTF here for the states I'm visiting. The locals will know about cool things that a Google search might not show you.