I also use critter-keepers for new froglets, with the vents taped across with clear shipping tape. I like these better than a shoe box because you can see what's going on, watch them feed, and the keepers usually have a central hinged door for feeding, so you don't have to worry about where the frogs are when you open an entire top. Along with a few plants cuttings, such as the pothos you are using, I also put an open shipping cup on its side with a little water in it and some smaller, finer plant cuttings. Java would be fine for this, as well. Very few flies are likely to drown in this, but if the frog feels like soaking, it can. Some froglets like this, but others would rather get under it. I sure wouldn't KEEP them in a closed shipping cup, however, not that you ever intended to. (I wonder where that recommendation came from?) You still need to mist every day or twice a day.
You can get a home garden sprayer (2.5 pt., or 40 oz) RL FLOMASTER from Walmart garden center. This is a pump-action sprayer that can put out a very fine mist and does a better job than the regular hand sprayers designed after counter and window cleaner spray bottles.
Those are sure tiny fellows. Most breeders I know don't ship them that young. Don't be in a hurry to move them to bigger tanks until they grow quite a bit, are obviously eating well, maybe 6 to 8 weeks.
It sounds like you're off to a good start and doing things right.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus
2 D. azureus
4 P vittatus