Impetigo with secondary bacterial infection: Given the presence of impetigo and a rash under the armpit, it's likely that the blister is also infected with the same bacteria, such as Staphylococcus aureus or Streptococcus pyogenes, causing the painful fluid-filled blister.

Eczema herpeticum: This is a complication of atopic dermatitis (eczema) where the skin becomes infected with the herpes simplex virus, leading to painful blisters. The presence of a rash under the armpit could be indicative of eczema.
Varicella-zoster infection: Although less common in this age group due to vaccination, chickenpox (varicella) can cause painful fluid-filled blisters and is accompanied by a rash, which could also be present under the armpit.
Bullous impetigo: A form of impetigo that causes blisters or bullae, which can be painful and fluid-filled, often due to Staphylococcus aureus infection.

Staphylococcal scalded skin syndrome (SSSS): A serious condition caused by staphylococcal toxins that can lead to widespread blistering and peeling of the skin. It's crucial to consider this diagnosis due to its potential severity and the need for prompt treatment.
Toxic epidermal necrolysis (TEN): A life-threatening skin condition usually triggered by medications, leading to widespread skin necrosis and detachment. Although rare, it's critical not to miss this diagnosis due to its high mortality rate.

Pemphigus vulgaris: An autoimmune disease causing blistering of the skin and mucous membranes. It's rare in children and would be an unusual cause of a painful fluid-filled blister on the foot.
Epidermolysis bullosa: A group of genetic conditions that result in fragile skin and blisters. While it could explain the blister, the presence of impetigo and a rash under the armpit makes this a less likely primary diagnosis.