A cough that occurs only during sleep is typically caused by allergies, post-nasal drip, or mild asthma-related airway sensitivity, rather than a bacterial infection. Cefixime (the active ingredient in the medication you mentioned) is an antibiotic and is not suitable or effective for these types of coughs. Using antibiotics unnecessarily will not help and can cause side effects. I recommend consulting a pediatrician to evaluate her chest and determine the exact cause. They can prescribe the correct treatment, which may include simple home remedies, anti-allergy medications, or inhalers, rather than an antibiotic.