Hello and Welcome to ‘Ask A Doctor’ service.
I have reviewed your query and here is my advice.
Oily skin differs from person to person and she 11 year old girl is in puberty
the sweat glands will act differently as per the hormonal
changes she is getting as per the age. Not nothing to worry unless this is causing problem.
As you have correctly mentioned it is possible that she is not cleaning the hair and scalp
applications properly. The lesions in the scalp growing down the face may be infected causing secondary infection of the lymph
nodes. If this is so, she needs to see a Doctor for clinical evaluation, examination and a course of an antibiotic and anti-inflammatory medications. If anything else is suspected then an ultrasound
and FNAC are also warranted. She she avoid hot and humid air, noon-time physical activities to reduce the sweating.There are no known ingredients that can reduce oil in sweat or sweating.
Hope I have answered your query.
Let me know if I can assist you further.