。
Tears ran down her cheeks.
Literal
Tears [subject-が] her [of-の] cheeks [in-に] flowed.
Just 頬に流れた ('flowed on the cheeks'), where に marks the location of the flowing. A simpler framing than the more vivid 頬を伝って ('tracing the cheeks'), which uses を to mark the path traced — but still acceptable.