Thank you for your photo and question. I agree with Dr Colin Theng that you should bring your child to a doctor or dermatologist to determine the cause of the blister and treat it appropriately.
The most common cause of such blisters seen in young children would be an insect bite reaction, although other causes such as infection, allergies or burns can certainly result in a similar appearance.
The treatment would usually consist of letting the fluid out of the blister and treating with an appropriate cream. Symptomatic treatment such as anti-itch syrup can also be helpful in preventing her from scratching the area.
Hope she gets better soon!
Warmest regards
Dr Stephanie Ho, Dermatologist
