It sounds like it could be croup, it's been going around, both my kids have it at the moment. If it is croup then the heater is actually a bad thing. The best treatment for croup (which may be a good test to see if it is croup and not something else) is to take her outside into the night air, rugged up of course. About 20-30 minutes in the cold air should reduce the inflammation and stop the coughing if it is croup. If it has not effect or makes it worse than you know you need to be investigating other causes like asthma or whooping cough. A steamy shower is the 2nd best option, but not nearly as effective. We have the vapouriser on at night and the window cracked about 3cm to let in the cold air, both kids are sleeping much better.