I am not sure what criteria one is supposed to use when rating a lava tube, but my family (which enjoys hiking and outdoor activities) had a surprisingly good time. The hike to the end of the underground tube (one mile) and back was the highlight of our Oregon trip for my six-year-old. I can see where some kids might be scared by the experience of hiking in a cold, dark, and confined space, but mine thought it was great. It's not a long walk, and you can always turn around early if you don't want to go all the way to the end.
You can rent lanterns onsite for a reasonable cost of five dollars, but I would urge groups to also bring flashlights or head lamps or to rent more than one lantern. We had extra lights, and I suspect that it would have been difficult for our group to have made it through with only a single lantern.