The Department of Conservation (DOC) suggest allowing 2 hours 30 minutes for the return walk along an easy graded gravel track that you could get a pushchair along if you were keen. Cyclists are also permitted to use the track up to the Nikau loop which would leave only a couple of hundred meters walk to the cave.
There are actually two caves here. The main one is 30 m long and no crawling required! There is often water on the cave floor so expect to get damp muddy shoes but the kids will love it. I haven’t been through the smaller cave but I understand from the sign that it is a tight squeeze.
Have a read of the DOC sign close to the cave entrance before you go in; it has all sorts of useful info on it. To find the cache make your way through the main cave until you arrive at the ladder. At the top of the ladder take two steps to the left and look for small crevice. The cache is a 200 ml sistema.
Have fun and don’t forget to bring a torch.