temp control in the toy hauler garage for kids

A 3rd A/C will take care of the summer heat. A space heater can be used to supplement the heat when it's cold out. (assuming you have hook ups)
If not be prepared to bundle up. In general the unit and ducts are not insulated well enough to keep the rear as warm as the rest of the unit. My experience has been the heat has cooled by the time it reaches the rear of the trailer