the are usually safe the only way they wouldn't be is if they had some type of contaminated wood to eat before hand.
they eat the cellulose out of plants so find a well rotten tree stump. the stump should crumble and breack very easily with your fingers. these will absorb water like a sponge try to get both large peices and small
the smal peice you should crumbly til they are fine powder like pour some in the bottom of the container then at some of the large chunks and cover them with more of the powder,you can place peices of plants and leaves in as you go also. the top the contain with a few flat peices of wood.
also i forgot to add that the wood needs to reamian damp you can even let some water pool in the bottom
pill bugs are relitives of crabs belive it or not some they are found near wet areas and need water to breith if they dry out they will huttle together to conserve water.
i feed wc ones to my chams and anoles they relish them and sprit to eat them i bet your frogs will do the same
...i have other info on growth and breeding too...