Sinusitis means "inflammation in the nasal sinuses" and is usually caused either by virus infections, like colds or flu, or by allergies. There is no treatment for virus colds, other than temporary relief of symptoms. There are many treatments for allergies, which can be tailored to meet the specific needs of each patient by the doctor.
Sometimes, children get a bacterial infection in their sinuses. It is not easy to diagnose this condition, since cultures will always show growth of common normal germs, such as strep pneumo. Sinus xrays are not helpful either, because they usually show sinus fluid and inflammation during normal colds, so there would be no difference on xray to distinguish a bacterial infection.
Green nasal mucus is a normal phase of a virus cold.
The diagnosis can only be made clinically, by meeting one of two criteria. The first is termed "severe acute sinusitis", and consists of thick pus-like nasal discharge, high fever, headache and sinus tenderness that lasts at least 4 days. In a regular cold the fever usually goes away after 4-5 days (or sooner), and the greenish mucous usually signals improvement in other symptoms. Severe acute sinusitis is sometimes called purulent rhinitis, but should not be confused with the normal greenish nasal discharge that is part of a regular cold.
The second and more common type of bacterial sinusitis is termed "persistent acute sinusitis". This means it must persist for over 10-14 days, without starting to improve. Most colds will start to improve before 10-14 days. They might hang on for several weeks, but at least be starting to improve by 10-14 days. Symptoms of persistent acute sinusitis include thick nasal discharge, cough both day and night, bad breath, but not usually much fever.
A cold that does not improve after 10-14 days should be treated with an antibiotic.
Many doctors treat colds before 10 days, trying to prevent or "head off" a bacterial sinusitis. This is a very dangerous practice, since it has no effect whatsoever on preventing sinusitis, but is one of the major reasons for the serious problem of bacterial resistance. (Be sure to read the health-byte on "bacterial resistance"!!)
Sinusitis is usually caused by the same bacteria that cause ear infections, so similar antibiotics can be used. They are usually used for longer courses because of the difficulty in getting the drugs from the bloodstream into the nasal sinuses.