Medicines like Acetylcysteine are not recommended to be used in children under 2 years of age. This is because the medicine works by dissolving the mucus which can cause a block in the breathing airway, and children below 2 years of age may not be able to clear it or cough it out properly. Still, in certain conditions it can be prescribed by your doctor, hence, follow the instructions as provided. Consult the doctor for any query.