Hello,
Thanks for posting your query.
Considering the information you have provided the most probable diagnosis seems to be a dermatitis or an allergic reaction.
Less likely allergic angioedema or a herpes viral infection might present in a similar way however these may be associated with severe swelling ( angioedema ) or significant painful blisters ( in herpes Labialis ).
Possible factors for triggering this condition are lip balms, lipstick, toothpaste, mouthwash, bubble gum (ie, rosin, cinnamates), nickel in musical instrument mouthpiece, rubber in snorkeling mouthpiece, cane XXXXXXX in a clarinet, food (mango skin , processed foodstuff containing certain chemicals etc.
The most important thing is to keep the lips well moisturised drinking plenty of fluids and applying petroleum jelly ( Vaseline etc) frequently .
Avoid any other chemical products .
The next best step is to consult a dermatologist who can confirm the diagnosis and start further appropriate treatment like a course of antibiotics ( if inflamed ), antivirals local acyclovir ointment or oral acyclovir ( if herpes ) and mild steroid ( hydrocortisone ) if eczema ( dermatitis).
Please do not worry your child will gradually get back the natural colour of her lips once this scab ( dead skin) formed due to the rash resolves.

Hope I have answered your query.
Kindly accept my answer in case you have no further queries.
Wishing her a speedy recovery.
Regards

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