It looks like a patch of eczema to me. I would ask your doctor about a prescription-strength cortisone (there are several which are mild enough for a child's face), or else one of the non-steroid alternatives like Protopic or Elidel.
Take care.
Dr. Rockoff
My daughter used to get these mysterious rashes all the time! Always a concern until you figure out what's up. Sometimes fifth disease can display itself this way (followed by a lacy rash over the trunk), and it can take awhile to go away, too, but it doesn't really require treatment, eventually goes away on its own, and is nothing serious. Other viral illnesses also sometimes first show up as a rash (or a "viral exanthum" as one doc put it when *I* was the one with one of these awful things all over me!)
On the other hand, sometimes these things are just skin irritations. When nothing else was up--no fever, no symptoms but the rash, no "punky" look, no "off" behavior--I sometimes just put a little bit of plain vaseline (non-scented) on there to protect the cheeks from the cold and dry weather, and from further irritation due to rubbing, etc. If the vaseline trick works, it's probably just an irritated skin issue.
Best of luck--hope it's nothing major. A call to the pediatrician's office often will ease your mind.