Smallest possible worm bin

Absolutely possible. My smallest worm "bin" is one of those small critter carriers they sell at the pet store. Get a small container with ventilation, add the worms and some extra substrate (like coconut coir, shredded paper, etc). Keep it most but not wet. Even feeding them "worm food" isn't strictly necessary, but for your frog's sake u can feed the worms nicer foods.