I've heard that you can't let them dry out because they breathe through their skin to some degree. The substrate must be pretty wet but not standing water, are you sure you have it wet enough?
I've been really looking into getting a few Pacman frogs, I'm wondering if you could put them in a hatchling rack with no belly heat? Seems like if you had clear tubs it would be about perfect for them. My next rack will be an ARS-1075, you could hold up to 75 hatchlings or frogs in that rack! And changing out the substrate would be simple. I keep my reptile room at 81F, apparently that's a good temp for Pacman frogs.
http://arscaging.com/ars1075.html