I am new at fruit flies, but from what I read around here, I wouldn't feed your frogs just regular fruit flies. Not knowing where they're from, they could be carrying parasites or diseases. There are some pretty more experienced user on this site, and they will be able to give you a better answer.
From the time being, here's a link to the fruit flies section on this site:
http://www.frogforum.net/food-feeder...ing-101-a.html
Eric