Ngilgi Cave near Yallingup is the northern cave option in the Margaret River region, useful if you are staying around Dunsborough or pairing it with Canal Rocks and Cape Naturaliste. It has large chambers, stalactites, and an interpretation layer that acknowledges the Wadandi cultural landscape rather than presenting the cave as an isolated curiosity.
The cave is easier to fit into a northern loop than Lake Cave or Jewel Cave. It is still underground, still damp in places, and still involves stairs, so wear decent shoes. Children usually enjoy it more than they expect to.