Take the kids to the Grotta Gigante for a tour. On the tour, you'll be walking down in to a large cave underground.
The path is paved so you won't have to worry about wearing any special shoes. But do wear lace up shoes. Flip flops would be a bad idea.
No matter what the temperature is outside, bring a jacket. As you descend into the cave, it gets cooler fast.
The tour guides really do a good job of explaining what you are experiencing in the cave. The tours are in Italian and English. Other languages might be offered depending upon the guides and the people in the group.