Continue giving Ambrolite syrup as prescribed to help loosen mucus and relieve coughing. Ensure she stays hydrated, uses a humidifier or steam inhalation at night, and keeps her sleeping area dust-free. If the cough worsens, fever develops, or breathing becomes difficult, consult a doctor promptly.

Since your 18-month-old has had a chesty cough with rattling for 20 days despite taking Maxtra, Ambrolite, and Piriton, you should consult a paediatrician. The doctor can examine the child, possibly order tests, and recommend appropriate treatment to clear the infection and prevent complications.