Hi M.. My daughter, who is almost 4 yrs old, had a sinus infection last month. My Pediatrician said that sinus infections are more rare in children than adults, since all of the sinus cavities are not yet developed. (I think he said the sinus cavities will not be fully developed until age 6?)
My Pediatrician asked if my daughter's breath had been bad...and it had been noticeably terrible! That's a symptom of a sinus infection in children. And he said that the dark circles under her eyes were also a symptom.
If she's not improving, I think you may need to make a trip to the Pediatrician. I hope she feels better soon.