I enjoyed Wind Cave, but it definitely is not for those uncomfortable in small spaces. At one point on our (walking) tour, the guide turned the lights off for a few seconds, and it was completely black. As I recall, there were also some places which were wet and somewhat slippery.

Skim back through older posts on this forum, and I think you will see others suggesting that 2-3 days is more than plenty for places in the Rapid City area (Rushmore, Crazy Horse, Wind Cave, etc.) I agree with that general notion. It is a good place to drive through but you (or your kids) may be bored out of your mind with a full week there.

Look at AAA's tour guide for suggestions for motels (in Keystone you will be very limited for hotels). You might also look at B&B's.

Two hours at Devil's Tower would probably be fine. I stopped by en route from Keystone to Denver, all within the same day, without any problem.