。
We've known each other since we were kids.
Literal
We [topic-は] child [of-の] time from [of-の] acquaintances are.
子供のころ ('the time when [one was] a child') — ころ ('time, period') is a temporal noun that attaches to clauses or nouns via の. 子供のころから = 'from childhood.' 知り合い ('acquaintance, someone one knows') is the masu-stem of 知り合う ('to get acquainted') used as a noun. The whole phrase is an elegant way to claim a long friendship.