Your librarian can help with suggestions, just ask. I agree that HP and Kingdom Keepers are good choices.
Generally speaking, YA titles on themes that are more "adventure" oriented will be good choices that are fine for kids to hear, and that won't bore the stuffing out of adults (never a good idea when you're driving.)
YA titles that have "coming of age" themes can be a bit problematic, as the authors like to make the angst realistic, which usually means that there will be some rough language and themes that might be inappropriate for younger kids, such as substance abuse and sex.
I also love audio books for long drives. I'm partial to humor titles and very action-packed thrillers, because those work best to keep me alert. (Getting caught up in "what's going to happen next?" keeps your adrenaline up a bit.) Bonus points if I can find a novel set in the area that I'm visiting: for instance, I *love* Randy White's Doc Ford novels on drives to Florida. (Those generally are not kid-friendly, though.)
Oh, and along the lines of the Great Courses idea, downloading TEDTalks podcasts can be very good as well. The stories and the style tend to be a bit more breezy and approachable.