My Daughter Is 6 Years (normally 19.3kg But Now Weighs

My daughter is 6 years (normally 19.3kg but now weighs 18.1kg due to illness), has been prescribed prednisolone sodium phosphate o-solon 5mg/ml (predmix oral solution). The dosage prescribed by after hours doctor was take if necessary once a day 1.8 ml. She is coughing all day and night and experiences tightness and some breathing difficulty in some coughing fits. She has been prescribed this before and the dosage with our usual doctor was 3 ml. Which is correct?