。
She had a happy childhood.
Literal
She [topic-は] happy young-years-period [object-を] spent.
幼年時代 ('childhood years') uses 幼 ('young, infant') + 年 ('years') + 時代 ('era'). It's a slightly literary alternative to 子供時代 ('child days') or 幼少期 ('early years'). 過ごす ('to spend, pass time') takes the period as an object with を — common with 時間, 日, 年, 一日, 時代. The verb is more reflective than 暮らす, often used in retrospection.