I think the length of your stay will be about perfect in each area.
Now that I have a better idea of your timeframe, I'll give a few more specifics you can look up on the web.
Look into the Harney Peak Hike like I previously suggested, it's not a streneous hike and kids can do it. Just bring some snacks and water.
I'd either head South and look into a tour at Wind Cave in Wind Cave National Park (Boarders Custer State Park to the South). Call ahead for reservations if you want to go see any of the larger caves. After or before the cave tour, look into spending an hour or two at the Mammoth Site in Hot Springs, SD.
Or you can head the other way West of Custer and see Jewel Cave. Hill City and ?? can't think of the other little town? Neat little tourist traps. I can't say I was impressed with Bear City (Bear Something??) or Reptile Gardens, but other people I know liked them. The Cosmos is worth the price of admission though and if your family likes go carts, check out "The Ranch".
There's a few dinner theaters in and around the area that are usually enjoyable. The Black Hills Playhouse is also something that I always enjoyed.
That's about 20 minutes from Blue Bell up near the start of the Needles Hwy.
Up near Deadwood, make sure to drive through Spearfish Canyon. I've had the chance to ride horses in and around that area as well and its gorgeous. There's alot of little things to do in the smaller towns. Various Train Rides, mine tours, and such, or just go hiking/biking.
Badlands, bring a hand held GPS if you stray off the trail (which I recommend doing). Be carefull though, as the terrain is rough and somewhat slippery. Your not allowed to pick up fossils
but some major finds have been found by just random hikers. If you look for areas that look like old dried up stream beds there are litterly hundreds of fossil fragments with the occassional tooth or skull caps and such scattered around.
A few hours in Wall Drug which is about 15-20 minutes away is worth exploring as well. I'm presuming the hotel you'll be staying in is there anyways, as there isn't much else in the area.
Anywhere you hike, always keep half an eye out for the possibility of a rattle snake. In 15 years of going out there, I've only scene a handfull of them, but they are there so be carefull.