Scarlet Fever: This condition, caused by Group A beta-hemolytic streptococci, is characterized by a bright red rash that covers most of the body, a red, bumpy tongue (often referred to as a "strawberry tongue"), and a sore throat. The presence of a fever, papular rash, and the specific mention of a red throat and white tongue (which could evolve into the strawberry tongue) make this a highly plausible diagnosis.

Viral Exanthem (such as Roseola or Enteroviral infections): These are common in children and can present with fever, rash, and sometimes pharyngitis. The rash and fever could fit several viral exanthems, but the specific combination with a red throat and white tongue might be less typical.
Kawasaki Disease: Although more commonly associated with a high fever, conjunctivitis, changes to the lips and oral cavity, and cervical lymphadenopathy, some cases might present with a rash and could be considered, especially if other symptoms develop.

Streptococcal Toxic Shock Syndrome (STSS): A severe condition that can arise from a streptococcal infection, characterized by severe pain, high fever, vomiting, and potentially a rash. It's critical to identify and treat promptly due to its high mortality rate.
Meningococcemia: Although less likely given the description, this condition can present with a rash (often petechial or purpuric), fever, and can be deadly if not treated immediately. The absence of specific neurological symptoms or a more characteristic rash does not rule it out entirely.

Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus, it can present with fever, sore throat, and sometimes a rash, especially if the patient is taking ampicillin. However, the age and specific combination of symptoms make this less likely.
Rocky Mountain Spotted Fever (RMSF): A tick-borne illness that can cause fever, headache, and a characteristic rash. Given the geographical and seasonal variability of tick exposure, this would be less likely without specific risk factors or exposure history.