Thank you for your query on Healthcare Magic.
This collection of fluid in her thigh may be due to infectious conditions like abscess
or an underlying bone infection
But without a detailed history and physical examination of the child, coming to a conclusion is very difficult.
Is the swelling of recent onset or since birth?
Is child having fever or pain?
History is important to tell the type of swelling.
Therefore I advise you to discuss these issues with the treating Pediatrician
for proper advise and for management plan.
Hope I have answered your query.