。
She went into the forest to look for the missing child.
Literal
She [topic-は] missing-status became child [object-を] search-for-purpose forest [into-へ] entered-and-went.
Three packed grammar points: (1) ~しに行く marks purpose of motion, with the verb stem 探し ('search') + に + motion verb. (2) The relative clause 行方不明になった ('having become missing') modifies 子供 — Japanese builds noun-modifying clauses by placing the verb directly before the noun. (3) ~ていく ('go and do') is the directional auxiliary, contrasting with ~てくる; here it depicts motion away from the speaker's vantage. 行方不明 (literally 'whereabouts unknown') is the standard term for missing-person cases, used in news and police bulletins.