OMG, this is one of those worst-kept-secret-locations that I did no know about, had seen many pictures of, but did not know where it was until I researched it. It is about a 20 minute walk from the carpark within Centenial Glen and has been known as Fairy Grotto and Harpers Cave over the years.
There is a narrow slippery ledge that will keep you dry and get you into the grotto itself, but you are about 5-6 feet above the water here, or, you can wade through the nearly chest deep creek to then climb up into the grotto itself.
I hope if you choose to visit this spot, you respect it and others that may be visiting, not like the large group of yobbos that turned up and did not care about anyone else and just wanted to make a lot of noise.
Anyways, ok for kids, though they may have issues getting into the grotto itself. Plenty of stairs lead down from the carpark into Centenial Glen, so, a bit of a huff n puff getting back out.