If your child has been coughing for three weeks after having a cold, it may be a good idea to consult a healthcare professional. In the meantime, one way to help reduce coughing in children is to use a saline or distilled water nasal spray to help clear mucus from the nose and throat. This can be done multiple times a day, following the instructions for the appropriate amount of water to use based on the child's age. Many parents have seen improvements in their child's symptoms within 48 hours of using this technique.