In my experience temperate zone mosses are seasonal and may not last long in viviarums, but it's worth a try to see what happens, You can always remove them.
If you collected them in a pesticide/fertilizer free zone, they'll be safe enough for your dart frogs, and probably your plants, as well. They won't contain any parasites that will affect your darts, but you may get something like slugs that will eat foliage, or possibly some sort of nematode that may attack the plant roots, but this isn't all that likely. It may even contain some interesting bugs for the frogs to eat. About the only way I know to treat the moss to kill all of the bugs is to seal it in a container with a piece of dry ice, which will fill the container with CO2 as it vaporizes. Or you can put this in your vivarium after it's planted and before you introduce the frogs, of course. This will kill about anything that breathes oxygen, including beneficial soil insects and arthropods. I've never actually done this myself, but it is an option.
There are a lot of people who disagree with my casual approach about not sanitizing everything I introduce into my tanks. I'm actually more careful about plants and wood I get from a nursery source, and do try to wash them off carefully. I'm more concerned with the artificial fertilizers and pesticides (or true plant pests) they may contain than with anything from the wild. Over the years I've been in the hobby, with about 15 vivariums now, only two pesty concerns have come up. The major one has been slugs--which can be controlled somewhat by simply picking them off as you see them, and a scale insect on a bromeliad that I introduced into one tank.
Hubris-- As soon as I brag about anything like this, something is sure to come up next week and make me sorry.
-----
Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
7 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
6 P. terribilis mint and organe
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus
2 P. lugubris