My child has a rash on his cheeks that looks like he was slapped? What caused this?

Many causes. There are many causes, but the one you're probably thinking of is fifth disease, or parvovirus b19. This child should be seen by their physician.
? Fifth Disease. There are several common rashes and infections which can cause reddened cheeks. The most famous rash for causing the "slapped cheeks" appearance is fifth disease. Fifth disease is caused by the virus commonly called parvovirus b19 (erythrovirus). It is a very common infection of childhood and is generally mild and self limited. It can cause miscarrages in women if infected during pregnancy.