Dumpy tree frog? What am I doing wrong?

[wriggles]

That's unfortunate =( They definitely don't look like dumpy tree frogs, in fact I wouldn't be surprised if they came from your neighbors bird bath lol. Our local pet shop is pretty pathetic when it comes to frogs too...

Unless your house is freezing cold I'd definitely turn off that heater for starters, 85 is pretty hot. If the top screen is covered, uncover it so the frogs get decent air flow.
How many frogs were in there? If the larger frogs see the babies move around, they're going to jump at them thinking they're food and could injure/eat them.
They would really appreciate more perching/hiding spots as well, it's a bit bare in there.
Make sure the water level doesn't get high enough to wick into the coco fiber or it'll get waterlogged and cause potential bacterial infections.

Other than that, they could have parasites or some other pathogens killing them. The owner could have been filling their water dish with unsafe city water as well, though I'm not sure how fast that would cause death.
Turn off the heat for sure though...