Sorry I'm a week late to answer this...was on a trip of my own!
I went to school in Richmond and worked in Williamsburg. Still have friends there that I keep in touch with. It IS going to be hot and humid, and if you are doing stuff outside, especially at dusk, be prepared for bugs.
Kings Dominion is just north of Richmond up I-95. In Richmond itself, I would suggest taking the kids to Maymont Park, which is a huge FREE park with an aviary, a Japanese garden, and lots of space. Here's the link:
http://www.maymont.org/Page.aspx?pid=308
Maymont was my favorite place to go in Richmond!
There is the Richmond Motor Speedway if you're into racing and there's a race while you're there. And there is a Children's Museum in Richmond. Here's that link:
http://c-mor.org/
Williamsburg is an hour east on I-64. Keep in mind that historic Williamsburg is a big summer tourist attraction and you will have to deal with the crowds (just like at the amusement parks). Williamsburg also has Busch Gardens as well as Water Country USA water park.
I would do a Google search for things to do in Richmond, VA (same for Williamsburg) or Google the Richmond, VA board of tourism. You asked about things to do along the way. I would go to www.mapquest.com and get an idea of what route your family is going to take and then do a Google search for things to do in any of the major cities you will pass through.
As for the heat and humidity, yes, it will be humid there in July. Maymont park has lots of shady areas. Of course the theme parks will have cool down misting stations and lots of air conditioned places to go inside as well as cold treats. I'm not sure about colonial Williamsburg since it's been a lot of years since I visited.
Good luck and I hope you have a fabulous time!