Try using a non perfumed washing powder for her bedding- like Surcare. Give her bed a wipe with a damp cloth and the mattress a good hoover and cover with a hypoallergenic mattress cover and the same for her pillow and duvet - give them both a hot wash as well. Make sure the room is not too warm, ventilated, as dust free as possible and place a bowl of water near the radiator to moisten the air. Either take her soft toys away, or give them a hot wash and /or a visit to the freezer in a plastic bag overnight. This should ensure that that dust mites and other common allergens are reduced and that she has a comfortable sleeping environment. If that doesn't make any difference and she keeps having the cough, go back to the docs again.
I don't mean to imply that you have a dirty house by the way, it is just that my oh has a dust mite allergy and and an allergy to any perfumed products (among other things!) and we find that these things help him and also soft toys are notorious for harbouring dust - you should also try to keep them to the foot end of the bed if at all possible.
Hope this helps.