Came across this little gem of info last night on the wb.
For relief from a persistant cough at night, put "vicks" vapour rub on the soles of the feet, cover with socks.
I imagine this would help in day time if you can get your child to stay off their feet.
Anyway, we tried it on DS2 who is almost constant coughing at the mo and it really helped.

Hope it can offer some relief to others