Dart frogs require a forest floor type substrate which would normally be soil, moss, leaves and wood / trees.
When replicating this we would normally use a mixed of soils and coco bricks to aid in soil drainage so your enclosure doesn’t get too water logged and can drain fast. Some products I have personally been using and had great success are the following:
I normally mix the “Earth Mix” & “Forest” together half and half depending on the size of the enclosure. I’d also break up a coco brick and mix it in too to help with drainage. When i’m building an enclosure I try to aim to have at least 5cm deep of forest mix through out.
I’d also add live moss and leaf litter to add humidity as well.