They will eat anything that moves, just like most frogs. Mine were eating f/t silversides and krill from tongs before they got sick along with crickets and earthworms.
My experience says something like the following is mostly going on: juveniles in their first year are captured somewhere opportunistic shortly before going into hibernation or aestivation at the same time each year. Chile is in it's winter now, and the high temps for Concepcion Chile three weeks ago were in the low 50'sF. I have found one note of capture in Concepcion, that is how I new to check the temps there.
Any claims of cb animals are, in my opinion, bs. Just because the animals are small doesn't mean they are cb. If someone was breeding them, finding info on their care would not be the needle in a haystack that it is now.
I have heard the Chileans farm breed them for food, but have found no reliable reports of where, how, or anything else. Three or four dealers bring them in each year at the same time and they sell fast. They are not expensive, I paid $22 each for mine.
I am really starting to lean towards the possibility that these frogs need real cold, super clean water. After all, they only come from Chile, and maybe northern Argentina, although no one seems to know this for sure. Coming only from such a thin strip of land (Chile) between Mountains (Andes) and an ocean says to me specialization.
Below is a great picture of an adult female. If you consider that the planks on the decking are most likely four inches wide...that is one monster frog.
http://www.generalexotics.com/images/chiliean-widemouth-frog.jpg