Hi.. Welcome to HCM.. So sorry to hear about your daughter s problem and will try my level best to resolve it to the best of my knowledge and experience.. As per your query your 9 years daughter who has her tonsils and adenoids removed is suffering from bad breath from mouth and also have a stuffy nose.. Yes, the symptoms can be due to Sinus infection and mucous accumulation in sinuses can cause stuffy nose because the sinuses drains through nose.. Bad breath and bad smell from mouth is a common complaint seen with sinusitis patients.. I would suggest you to consult a Paediatrician or an ENT Expert and get her evaluated and a thorough clinical evaluation and investigations like PNS viw x ray of sinuses or MRI scan can help in diagnosis and treatment can be done as per the confirmation.. In case of sinus infection a course of antibiotics, anti inflammatory painkiller like Ibuprofen or Naproxen, decongestant nasal sprays for relieving stuffy nose, saline nasal irrigation and steam inhalations can help.. Inhaling steam with few drops of eucalyptus oil in boiling water can help in refreshing breath.. Hope this helps.. Regards..