Healthy sinuses are normally filled with air, but when we get a cold or suffer from allergies, they can become full of mucus. If bacteria grow in this mucus, it causes a sinus infection, defined as painful swelling of the sinuses and nasal passages. Sinus infections shouldn't happen often, so if your child seems to get them all the time, talk to your pediatrician. Chronic sinusitis can be difficult to diagnose in kids since the symptoms are often similar to upper respiratory infections like the common cold -- including green discharge from the nose, cough, headache, and fever -- which young kids are especially prone to (some children have as many as six to eight of these upper respiratory infections a year).
To see whether sinuses are to blame, doctors look at the duration of the infection, rather than just the symptoms or frequency (a cold may last a week; a sinus infection can hang around for two weeks). Sometimes chronic sinus infections are caused by allergies, so you may be able to prevent future infections simply by treating the allergies.
If no allergies are found however, your pediatrician may refer you to an eye, ear, and throat specialist (known as an ENT). This doctor will probably first want a detailed history of your child's symptoms, after which he'll examine your child's ears, nose, and throat. Sometimes a nasal endoscope (a skinny telescope used to examine the inside of the nose) is needed to see farther into the nasal passages, but don't worry -- it isn't too uncomfortable, and many kids even think it's cool. Sometimes an x-ray or CT scan is also done to check out the sinuses (usually only in kids over 6).
Sinus infections are usually treated with antibiotics, but if your child has had several (six or more) over the last few years or has chronic symptoms that never seem to get better, your ENT may decide that a minor surgery is a better treatment option. Your doctor may opt to remove your child's adenoids (glands located in the space between the nasal cavity and the throat), which are often the source of sinus infections.