。
She gave the cat some milk.
Literal
She [topic-は] that cat [to-に] milk [object-を] gave.
やる ('to give') is the rough or down-status counterpart to あげる — used for animals, plants, and traditionally for inferiors or close family. With pets it's the natural choice: 犬にえさをやる ('feed the dog'), 花に水をやる ('water the plants'). Using あげる with animals is becoming more common as people anthropomorphize pets, but やる stays the unmarked option. に marks the recipient. The simplicity of the sentence — clean SVO frame, plain past tense — fits a casual narrative voice.