My 6 year old daughter is fighting a cold. she now has a cough. would leaving a cool mist humidifier on throughout the night help her?

Probably.: Cough may be worsened from the irritational effect on the respiratory tissues by heating the air in our homes at night; so, the moisturizing & soothing effect of a cool mist humidifier could prove useful. Hot vaporizers can be potentially dangerous by causing scald burns if accidentally tipped over by an unsuspecting toddler. Just another reason to prefer a cool mist humidifier. It's worth a try!

Maybe,maybe not: These help most when the environment is very dry, like in winder, when the heater pulls the humidity down below 20%.Humidifiers & vaporizers can help bring up the humidity up where it is not further drying the kids airway when they are sick. If the basic humidity the kid is in runs 30-40%, the humidifier has less positive benefit.