From what you are saying, you may need to look into your tank size vs heater wattage. I try to keep at least a 200W heater for my 46G tank. Anything less would strain the heater, especially during water changes. I find that with Discus, you really don't want to cheap out on the heater, get a quality one that can keep up with the size of the tank.
As far as the meds and disease, cotton mouth could be fungal, or bacterial. Either case, increase in temp for bacterial infections is a no-no. Bacteria enjoys warmer temps and higher PH. If your water is hard, and the temp is raised during this period, it would additionally stress the fish. The addition of EM has a negative effect on your bio-filter, so be ready to test for spikes. Have some water conditioners handy in case you need to do water changes.
As much as your child loves the fish, it's both dangerous to the fish, and him should the glass fail and break. My nephew learned through strict discipline that there are things he must not do, one of them is slapping the tanks.
Good luck!