Welcome to health care magic forum
Thank you for your query
I understand your query regarding your daughter's skin problem
It may be due to sun exposure ,acanthosis
or allergic condition.
It is possible to have more tanning
over particular one area.
she need to apply good sunscreen lotion twice a day.
it is better to consult dermatologist
weight gain is also reason for darkness,when it is associated with some hormonal
do not rub or scrub over the affected area.
I hope I answered your query.