Contact Dermatitis: This is a common condition in toddlers, often caused by irritation from diapers, wipes, or other substances coming into contact with the skin. The unilateral presentation could be due to asymmetric exposure or sensitivity.

Eczema (Atopic Dermatitis): Toddlers with a history of eczema can present with papular, red rashes that can be unilateral, especially if triggered by specific allergens or irritants.
Impetigo: A contagious bacterial infection that can cause red, papular rashes, often starting in one area and potentially spreading. It's common in children and can be unilateral in its initial presentation.
Diaper Rash (Irritant Dermatitis): While often seen in the diaper area, an irritant dermatitis can sometimes present unilaterally if the irritation is not evenly distributed.

Child Abuse: It's crucial to consider non-accidental injury in any child with unexplained or unusual skin lesions, including unilateral rashes. Burns, bruises, or other injuries can sometimes present as rashes.
Insect Bite or Sting: Certain insect bites or stings can cause significant localized reactions, including papular, red rashes. Identifying the cause is crucial for appropriate management and to prevent future reactions.
Cellulitis: A bacterial infection of the skin that can present as a red, warm, and tender area. It requires prompt antibiotic treatment to prevent spread and serious complications.

Kawasaki Disease: Although rare, Kawasaki disease can present with a rash, among other symptoms like fever, conjunctivitis, and changes to the lips and tongue. It's a condition that affects children and requires early diagnosis and treatment to prevent cardiac complications.
Juvenile Psoriasis: Psoriasis can occur in toddlers, though it's less common. It may present with red, scaly patches that can be unilateral, especially in its initial stages.