hmmm, interesting. when I raised a bunch of wood frog tadpoles what seemed to work was piling up gravel at one end of the tank to make something more like a shoreline with the water reduced to about 2 or 3 inches deep. moisture, even on the above water part is pretty important. if you have the gravel gradually rise above the water then they can crawl out a little at a time rather than go straight from water to land as they adjust to the change. I'm not sure if it would make any difference at all but that's how I had it set up and they made it out alright.

I've dealt with a few juvenile gray treefrogs that wouldn't eat fruit flies at all -- the only thing I could get them to eat were live black worms available at pretty much any pet store. you might give that a shot as well all though it doesn't sound like they are starving to death.