You will just want the moss to be damp to the touch, not sitting in water. As long as the container is closed it shouldn't dry out too fast. You can mist it as needed but there shouldn't be any standing water in the bottom unless you have a drainage layer. I believe northern frogger has some videos on youtube that show his sphagnum based froglet growouts.
I do mine a little bit differently. I set up a latching steralite shoebox half way with ABG mix and use it as a large springtail culture first. That way there is plenty of food for newly morphed froglets. I raise tadpoles in Mason jars or deli cups until they start absorbing their tails. When I have tadpoles ready to morph out I put in a layer of leaf litter over the ABG and set condiment cups with the tadpoles that are absorbing their tails on the leaf litter. As the tadpoles leave the water I remove those cups. A few days after the first froglet leaves the water I add a small slice of banana to one corner of the tub and start adding a small number of dusted melanogasters fruit flies daily. After a month or two I move the froglets in groups of 3 or 4 to a planted 10 gallon growout tank with a drainage layer, and ABG capped with plenty of leaf litter. From there I move them to bigger vivariums as they approach maturity. Everyone does things differently so find what works for you.