Viral Exanthem: This is the most common cause of rash and fever in children, with a wide range of viruses such as enteroviruses, adenoviruses, and parvovirus B19 being responsible. The rash can vary in appearance, and the fever is usually mild to moderate.

Bacterial Infections: Such as streptococcal pharyngitis or impetigo, which can cause a rash and fever. The rash in impetigo is typically vesicular or pustular.
Allergic Reactions: Including contact dermatitis or allergic reactions to medications, which can present with a rash and fever, although the fever is less common.
Insect Bites or Stings: Can cause a localized or generalized rash and may be accompanied by fever, especially if there is an allergic reaction.

Meningococcemia: A severe bacterial infection that can cause a rash (typically petechial or purpuric) and fever. It is life-threatening and requires immediate medical attention.
Kawasaki Disease: An acute febrile illness that includes a rash, fever, and other systemic symptoms. It can lead to cardiac complications if not treated promptly.
Rocky Mountain Spotted Fever (RMSF): A tick-borne illness that presents with fever, headache, and a characteristic rash. It is severe and potentially fatal if not treated early.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Severe skin and mucous membrane disorders usually caused by medications. They present with fever and a rash that can progress to blistering and peeling of the skin.

Juvenile Idiopathic Arthritis (JIA): Can present with fever and rash, although this is less common. The diagnosis is usually made based on chronic joint inflammation.
Lyme Disease: Caused by the bacterium Borrelia burgdorferi, transmitted by ticks. It can cause a distinctive rash (erythema migrans) and fever, among other symptoms.
Scarlet Fever: Caused by group A beta-hemolytic streptococci, it presents with a characteristic rash, fever, and other symptoms like a strawberry tongue.