Why would you need to do this? I would think that this would make your tank too hot. Most species of dart frogs prefere to be in the upper 60's for night time and mid 70's during the day. Any thing over 80 for long periods would most likely be detrimental.
The heater is not necessary. Room temperature is fine. A heater that is submerged in the drainage layer is just too risky in my opinion. In addition to the risk of overheating, you run the risk of it breaking under there introducing electricity.
The only time you'd really need to consider supplemental heating is if the temps get too low, like during the winter months, or say boosting the temp a couple degrees for breeding purposes.
You could accomplish either of those by other means, but if you're set on using an in tank heater, be careful in how you do it.
They're designed to be in contact with water. Their use as a substrate heater is questionable. How and where the temp sensor is placed will make a difference. How wet the substarte is, will make a difference.
They break. Some break in a stuck on condition until they burn out. This would be a bad situation. Having a backup to shut it off is a good, if not necessary, idea.
Frogs need to regulate their temps, and mostly do it by moving to environments with different temperatures. Using a heater that creates a uniform temp within the tank is not a good idea. Temp gradients within the tank need to exist.