Viral Exanthem (e.g., Chickenpox): This is the most likely diagnosis given the presentation of diffuse vesicular papules in a child without constitutional symptoms. Chickenpox is a common illness in children characterized by a vesicular rash that progresses from the trunk to the extremities.

Insect Bite Reaction: Children often experience insect bites, which can cause papular or vesicular lesions, especially if there is an allergic reaction to the bite.
Contact Dermatitis: This condition can cause vesicular papules due to an allergic reaction or irritant exposure, which is common in children who may come into contact with various substances while playing.
Eczema (Atopic Dermatitis): Although eczema typically presents with more chronic symptoms, acute flare-ups can cause vesicular papules, especially in areas prone to scratching.

Staphylococcal Scalded Skin Syndrome (SSSS): Although less common, SSSS is a serious condition caused by staphylococcal toxins that can present with widespread blistering and peeling of the skin. It requires prompt recognition and treatment.
Toxic Epidermal Necrolysis (TEN): A severe skin condition usually triggered by medications, TEN can cause widespread skin necrosis and detachment, which can be life-threatening if not recognized and treated promptly.

Pemphigus Vulgaris: An autoimmune disease causing blistering of the skin and mucous membranes, pemphigus vulgaris is rare in children but can present with vesicular lesions.
Bullous Pemphigoid: Another autoimmune blistering disease, bullous pemphigoid is uncommon in children and typically presents with larger blisters rather than vesicular papules.
Epidermolysis Bullosa: A group of genetic conditions that result in blistering of the skin, epidermolysis bullosa can present at any age, including childhood, with various types having different triggers and severity.