R.D.
There is a closer cavern called Skyline Caverns that I think is MUCH better for young kids. For one it's cheaper :) Also the tours are given by an actual tour guide not by headphones. The caverns are smaller and feel more like "exploring." Then there is a really fun mirror maze and a mini train ride. And the kids can touch one of the fallen stalagcites (sp?) I have done both and highly recommend Skyline. When they are older Luray would be a nice trip. Here is the link with coupons. If I remember correctly kids under 5 are free for the caverns.
http://www.skylinecaverns.com/discountcoupon.htm
Enjoy!