There are a number of reasons why a child would “cough constantly” at night including, but not limited to asthma, acid reflux (often referred to as GERD), chronic sinus disease and allergy to some inert substance or to an animal in her bedroom. In addition, a valid list of likely causes would have to take into account your daughter’s age.
The first thing is that you and her doctor work together to determine the cause of the cough. Propping her up” might help but that would not address the cause and you cannot and should not be expected to control her cough on her own. Only by diagnosing the cause will it be possible to apply specific therapy to best treat and control the cough and if her doctor is unable to determine the cause, it would be reasonable for you to request a second opinion from a respiratory specialist.
Good luck
Get a humidifier and if that doesn't work try taking her to the bathroom shut the door turn the shower on hot and the sink the steam will help alot