A small bowl would work, absolutely, as long as the frogs themselves can get in and out with success. Wesleybrouwer, I do agree that the dirt will likely not cause impaction, especially with adults, but the frogs are very young, and will not be able to perfectly use their tongues to catch food alone. If a young frog that size ingests the moss, it may cause harm. Not saying it will, but it's a greater risk with froglets.

Also, because this frog has been caught as a tadpole, he will be more calm and accepting of the fact that his owner is a person. Especially once he figures out that the soul reason for his handling is to get yummy crickets Once he's older, and has more control and coordination, it will be absolutely fine to feed him in the tank. The greatest problem with that is the water, and drowning crickets. He will get used to a bit of human handling to get food, as I said before.

This is, of course, just my opinion on the issue. Either method is effective.