Commonest causes of nocturnal cough in this age group could be asthma. If she does not have any cold then upper resp causes r less likely. Incessant cough only at night is mostly due to nocturnal wheeze. Kindly specify if there is such history in the past . She needs to be treated with bronchodilators and mast cell stabilisers if the clinical picture collaborates.

Your child may be suffering from postnasal drip. You may put one drop of Otrivin pediatric  in each nostril at bed time only for three days.