I was thinking I made a poor decision taking my 5 year old on a 4 hour drive each way. The trip was broken up very well with a visit to a supermarket on the way there and back, providing ample refreshment, food and bathroom opportunities. There was a very good documentary on the way up and a family-friendly movie on the way back to keep your mind off of the drive.
Our guide, Brian, did a wonderful job injecting trivia, making sure we hit the high points and helping with family photos. I think he's done hundreds (literally) of these tours - he had all the answers.
If I had to do it again, the only tips I'd have are 1) Weather - for some reason, I thought it'd be colder than it was. It was in the low 80s and a little cooler by the sequoias 2) Bring some other form of entertainment, if you have kids. My kid seemed to be occupied but we had a device with her preferred entertainment as well. 3) There are a couple of opportunities to play in the water if your kids want so bring water shoes and a towel if they're so inclined.
Thanks for a great trip, Brian!