Grandad used to take us here to look at fireflies too. The path in has recently been concreted, so this is ideal for families to arrive at dusk to see the annual phenomenon. The best time is the turn of season, between September and November. The fireflies appear just after nightfall. You need to turn off the torches (keep the kids out of the creek) and be patient.
The additional waypoints show where to park and where to wait for the fireflies. The cache itself can be reached a short stroll or rock hop (your choice) away.