Fruit flies is a froglet food you for sure should use. I use non-flying fruit flies, to feed I partly open the froglet tank (my one has flexible lid which makes possible rising it from one corner only), then I hit a few times the roof of fruit flies breeding jar to make flies fall down on the floor, then quickly open and pour out some flies into the frog tank. When enough flies are in the tank, quickly close the tank, put the flies jar into vertical stance and hit the roof a few times to make the flies fall down again, than quickly close the jar. Some fruit flies always manage to escape, it's not a reason to panic, they doesn't seem to be irritating neighbours and wild fruit flies often live in the houses, so letting them escape won't actually add something new to your enviorment.
The amount of flies should be given sounds as "as much as the froglets can eat". When that small, they grow actively, and can eat almost 24 hours daily. I doubt the possibility of overfeeding froglets on their first months of "frog life".
Please note that calcium is very important for froglets. Adult frogs usually fed calcium by sprinkling their life food with it. Sadly fruitflies are too small and putting calcium on them seems to be an insurmountable task to me. Possible solution is sometimes feeding froglets with baby crickets powdered in calcium, but I personally fill a large syringe with some calcium, then suck some water with it and shake it up to make some kind of calcium solution inside. This solution is poured onto the froglets to let them absorb the calcium with the skin. Not 100% sure this works as well as feeding calcium.

Hope this helps.