Control the fever. Roseola is a common viral illness in young children that typically causes very high fever, then a rash. The best treatment is to control the fever with Acetaminophen or Ibuprofen (for children 6 months and older). Encourage plenty of fluids and rest if your child seems tired. The rash is not bothersome and resolves on it's own. In general, the rash does not typically appear until the fever breaks.
#1: Don't Panic! Roseola is one of the least serious infections out there, but it can produce scary-looking thermometer readings (my personal record is 105.9 f). That's dangerous because a parent can have a heart attack looking at the thermometer; the only danger to the child is being overdosed with fever medicines because of fear of fever. Hydration, patience, and normal doses of Acetaminophen are all you need.