When the lost child saw her mother's face, she burst into tears.
Literal
Lost-child became child [topic-は] mother [possessive-の] face [object-を] see [when-と] suddenly-cry-begin.
わっと ('suddenly, with a sudden outburst') is a mimetic + と frame — わっ captures the eruption of emotion, と makes it an adverbial modifier. 泣きだす ('to burst into tears') is a compound verb, 泣く + 出す, marking the sudden onset of an action. Combined, they vividly portray a held-in emotion breaking free.