My pre-teen daughter and I hunted down the lava river cave after dropping my wife and son off to ski at the Snowbowl. Several miles up 180 and a couple of log roads later we arrived at the site and were happy to see that quite a few others had made the trek too. No ranger station, not much fanfare, no concessions, just a big hole in the ground surrounded by rocks and minimal signage.
Peering in, we could hear "wow", "cool" and "awesome" from the headlamp/flashlight clad visitors. Game on!
Getting into the cave is a bit of a challenge as it's quite steep, but at least there is some light at that point---and only that point. Once down the initial decent, the light disappears completely other than the occasional beams from the other visitors. It really was like walking on a waterless riverbed....sometimes in a tunnel that would fit a train, sometimes having to do a short crab crawl. It really was awesome.
Though it was cold in the cave (maybe 35-40?) I was sweating my rear off as we hiked the 3/4 mile to the end. It's not a loop, but an out and back. I wore tennis shoes, but wished I'd worn my hiking shoes as the riverbed is fairly rough. The tunnel is mostly very wide, no crowding or single-file other than a bit on the initial decent/ascent.
I saw people of all ages there, from 5-65. Our hike all the way through must have been about an hour and half, but you can really get a great feel for the wonder of it without going all the way to the end.
Well worth the out of the way drive. Bring some water for after your hike. Flashlight a must, headlamp better. My daughter's iPhone flashlight worked great and didn't kill her battery.