I find cough medicines pretty useless. If you think she will drink it you could try a bit of honey and lemon juice in warm water? I think it helped my DD when she had bronchitis. I made it up for her after dinner and she sipped on it until bed time. Or some just give a spoonful of honey before bed (make sure to brush teeth afterwards though!). Honey is supposed to be a natural cough suppressant.

Good luck, hope you all get some sleep soon