Wind Cave or Jewel Cave/SD

Can't help you with the caves, but there is a heck of good little dinosaur museum on main street in Hill City with a couple whole t rexes and then a lot of t rex heads big enough for an 8 year old to sit in. :)

Never been to Wind Cave but Jewel cave is a great place. We enjoyed, but no tykes with us.

Jewel Cave has more of what most people expect to find in caves: stalactites, stalagmites, cave bacon. Wind Cave is what is called a dry cave. Little water means no big formations. Lots of boxwork there, which isn't common in most other caves. Both are unique in their own way, but I might be more inclined to steer you to Jewel.

The Mammoth Site in Hot Springs is also a good spot to take an eight year old, especially if you can get in on the Junior Paleontologist Excavation. I don't know the dates on that for this year, but you could check the website if you are interested.

You need to get to Jewel early in the morning or you will be in for a long wait.

Wind Cave because it's different than most of the other caves. Agree on the Mammoth site in Hot Springs. Fascinating.

Check out the Candle Light tour at Wind cave. That's something he'll remember.

I did both within a few days of each other and been in over 20 other caves. My choice would be the one closest to your location or other things you want to see. Both are worth the trip and both have their unique things to see but neither worth driving extra distance to see one over the other.

If you are between the two, then I would say Wind Cave because there is more to see in the area.

Those who recommend seeing the Mammoth Site are spot on. That site draws paleontologist from all over the world. That is where I learned we had camels in the US at one time.

Went to both in 2013, enjoyed both. Either way, stop by the State Game Lodge in Custer State Park for a drink or dinner, and pay attention to the fireplace construction in lounge and bar. The beautiful rock crystals came from those caves, according to the bartender. (Game Lodge is also a good base for area camping.)