12" tall terrariums are not well suited for dart frogs. By the time you add in a drainage layer and substrate you've used up at least 4" of the space leaving only at max 8" of usable height for plants and frogs.
The two posts above me make excellent points. All I will add to it is that my terribilis climb regularly to heights of about a meter, and would go higher if the tank didn't stop there. Some climb more often than others but they all do. I'm a big fan of the 36" and higher tanks and wouldn't recommend anything lower than 24" tall for them.
I do use lower enclosures as grow-outs but like most frogs, P. terribilis grow very fast so wouldn't stay in a grow-out for too long.