I had success with treefrogs using aphids until they grew large enough pinhead crickets or 1 week old crickets. I turned them loose after that point. Not sure if you've got aphids, but I used to find tons of them on dandelion heads, so I'd just snip one off and put the whole thing in their enclosure.
Where they collected from the wild? How large are they? Do they show any indication of limb buds? [Be aware that some anurans remain larvae for quite some time.]
When they metamorphose, it would be best to feed them pinhead crickets (kits for culturing them are commercially available) dusted with an amphibian-specific multi-vitamin supplement.
I may be wrong but one thing to keep in mind is whether or not they will be aquatic after completeing the cycle of tadpole. I have a wet marshy area in my back yard and every year frog's lay eggs. If I am not mistaken once these frogs reach maturity they leave the water and start living on land. At least I think they do. It is very rare they complete the cycle before the water dries up. One year I actually kept a water hose running for them and that year I saw some of them hopping away from the puddle. They were only about as big as you pinky fingernail.
I would be prepaired for a wait, on tadpoles some may take up to 3 years to develope into frogs. and if you dont know what type they are they might not be frogs they may be toads