Formal:
* La jeune fille au chapeau rouge: This is the most literal and formal translation.
* La fille portant un chapeau rouge: This translates to "the girl wearing a red hat."
Informal:
* La fille avec un chapeau rouge: This is a more casual way to say it.
* La petite fille au chapeau rouge: This translates to "the little girl with the red hat."
Literary:
* La fillette au chapeau écarlate: This uses the more poetic word "écarlate" (scarlet) for "red."
You can choose the translation that best fits the situation.