Varicella (Chickenpox): Given the recent upper respiratory tract infection (URTI) and the appearance of perioral blisters (cold sores) followed by new maculopapular pustules on the face, varicella is a strong consideration, especially if the child has not been vaccinated against it. The initial presentation of cold sores could be a herald sign.

Impetigo: A common skin infection in children, often following a viral illness like URTI, which can present with maculopapular pustules. The fact that the initial lesions were perioral and are now spreading could suggest a secondary bacterial infection.
Eczema Herpeticum: Although the child is improving with Abreva (an over-the-counter antiviral cream for cold sores), eczema herpeticum is a consideration if the child has a history of atopic dermatitis. This condition is characterized by the spread of herpes simplex virus within the skin of individuals with atopic dermatitis.
Contact Dermatitis: An allergic reaction or irritant contact dermatitis could explain the maculopapular rash, especially if the child has been exposed to new products or substances.

Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): Although less likely, these are severe skin conditions usually triggered by medications or infections. They can present with skin lesions and are medical emergencies due to their potential for severe morbidity and mortality.
Infectious Mononucleosis: Caused by Epstein-Barr virus, it can sometimes present with a rash, especially if the patient is taking certain antibiotics like ampicillin.

Pityriasis Rosea: A skin condition that can follow a viral infection, characterized by a herald patch followed by a widespread rash. It's less common in this age group and the presentation doesn't perfectly match the described symptoms.
Kawasaki Disease: An acute febrile illness of childhood that can present with rash, but it typically includes other symptoms like fever, conjunctivitis, and changes to the lips and oral cavity, which are not mentioned here.