The Witcher Characters: Books Vs. Games Vs. Netflix Show
9. Ciri
There is an obvious age difference between Netflix and Witcher 3's young Ciri - a difference that likely stems from the show's attempts to make Sapkowski's world less horrible (as Ciri, even as a child, had certain inappropriate attempts made against by male characters in the books).
As for the differences in clothing, Freya Allan's Cirilla is either within the royal palace in Cintra or on the run, while the young Ciri in Witcher 3 is training with the witchers at Kaer Morhen; so each have attire to suit their respective contexts.
Ciri is introduced in the books as having "fair, mousy hair and huge, glaringly green eyes". And, at her first encounter with Geralt, she "could not have been more than ten years old" - hence her age in Wild Hunt.
For additional detail: her training outfit is described as "a sort of leather suit sewn together - or rather stuck together".