Infectious Mononucleosis (Mono): Although the rash is not a classic presentation, the fever, tender lymph nodes, and the progression of the rash could align with infectious mononucleosis, especially if Epstein-Barr virus (EBV) is the cause. However, the rash in mono is more commonly associated with ampicillin use.

Viral Exanthem (e.g., Roseola, Enterovirus): The fever and rash pattern, especially the rapid progression from the face to the trunk and extremities, could suggest a viral exanthem. These are common in children and can present with a variety of rashes and fever patterns.
Scarlet Fever: This condition, caused by Group A beta-hemolytic streptococci, presents with fever, a characteristic rash (which can start on the trunk and spread), and sometimes a sore throat. The rash is typically more erythematous and sandpapery than a macular popular rash.
Kawasaki Disease: Although the rash description doesn't perfectly match the typical polymorphous rash of Kawasaki disease, the fever, and potential for lymphadenopathy (if the suboccipital tenderness is due to lymph node enlargement) could suggest this diagnosis, especially if other criteria like conjunctivitis, lip changes, or changes in the extremities are present.

Meningococcemia: This is a life-threatening condition caused by Neisseria meningitidis. The rash can start as a pink macular or popular rash and progress to purpura. Given the severity and potential for rapid progression, it's crucial to consider this diagnosis, especially with fever and any signs of systemic illness.
Rocky Mountain Spotted Fever (RMSF): Caused by Rickettsia rickettsii, RMSF presents with fever, headache, and a rash that can start as macular or popular and progress to petechial. The rash typically starts on the wrists, ankles, palms, and soles but can spread. This diagnosis is critical to consider due to its severity and the need for prompt antibiotic treatment.
Streptococcal Toxic Shock Syndrome: This condition, caused by streptococcal toxins, can present with fever, rash, and severe systemic symptoms. The rash can be variable but considering the potential severity, it's a diagnosis not to miss.

Juvenile Idiopathic Arthritis (JIA) with Systemic-Onset: While JIA can present with fever and rash, it's less common and typically accompanied by significant joint symptoms. The systemic onset type can have a high fever, rash, and lymphadenopathy but is less likely given the acute presentation described.
Leptospirosis: This zoonotic infection can cause a wide range of symptoms including fever, headache, and rash. It's rare in children but could be considered in those exposed to contaminated water or animals.