My family recently visited Natural Bridge Caverns. We took the Discovery Tour, where you travel 180 feet below the ground. The length of the tour is 3/4 of a mile according to their website, but given the depth that you travel, it may be a bit strenuous for young children or those who are not in shape (the climb back up was a struggle for me). There are railways through most of the tour, but the pathways are slick (since it is a cavern, there is water that drips down), so I recommend wearing sneakers. The site says strollers are not recommended, and I cannot even imagine trying to navigate this course with a stroller. Some of the pathways are very narrow and at some points, there is a lot of winding going both up and down.
We had a tour guide for the duration of our 70 minute tour who was very informative. He gave us information about the history of the caverns, pointed out key formations during the tour, and gave information about how they were created. He was entertaining and engaging and very willing to answer questions. Although the tour is strenous, breaks are provided throughout the tour and there is bench seating available as well as a water fountain mid-way through the tour. You can also bring your own water bottles if you choose. No other food or drinks are allowed.
My kids absolutely LOVED the tour and spent a great deal of time researching the caverns and formations they saw afterward. If you're looking for a unique, educational experience for your family, this is it!