We do allow infants and even had a few just months old. We only front pack carriers as we want the infant's head in front of you. Most 3 1/2 years can make the tour with very little assistance or carrying. There are several different rooms that we stop and discuss the tour. This provides an opportunity for children and adults to sit down before heading to the next room.
It is safe as long as they stay with you. It is against the law to touch anything in the caves. There are around 600 steps up and down. They cannot leave the path, but it is not blocked off in any way.
Just did this last month. Boat ride, bus ride up hill, cave is cool, this one of the smaller tours. Shasta dam tour is good also
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