A stray dog suddenly pounced on the child.

Literal

Stray-dog [topic-は] suddenly child [at-に] pounced-on.

野犬 ('stray dog, wild dog') is the compound for dogs without owners, roaming loose. 飛びかかる ('pounce on, spring at') is a compound verb: 飛ぶ+かかる, with the auxiliary かかる adding the 'attack, come at' sense. Takes its target with に.