Yes, you can give Vicks Nyquil syrup to your child who is taking Novamoxin for sinusitis. However, Vicks Nyquil may contain additional ingredients for symptoms your child may not have. For cough caused by post-nasal drip, nasal hygiene with a saline solution like Hydrasense or Sinus Rinse is recommended. This can help alleviate symptoms and improve the effectiveness of the antibiotic. It may feel a bit uncomfortable but can work wonders when done regularly throughout the day and at least 30 minutes before bedtime. Feel free to ask if you have any other questions!