Agree with other posts - time of year and the fact you've only got a week is going to narrow your possibilities. Hopefully you'll have more time in the future to take your now-6-year-old to some of the most fantastic geology anywhere on the planet. From the one's we've visited, our "top five" would be:
Yellowstone
Crater Lake
Grand Canyon
Yosemite
Sequoia/King's Canyon.
Our "bucket list" is too long to post here. Given your limitation of time, I wouldn't confine my search to national parks. There's plenty of good stuff to do with a 6-year-old in other locations; for example Lincoln Boyhood Nat'l. Monument is adjacent to Santa Claus, IN and Holiday World. The latter would be neat for mom & dad, too. Check out Crater of Diamonds State Park in Arkansas. Digging in the dirt is always a big hit with kids, only here you/they can experience geology on a kid-friendly level. After you're done digging there's a neat little water park there, too. Doing the research is half the fun. Good luck, enjoy & safe travels.