N4
ているあいだに
While, During the time that ~
ねえ、私が数えている間に隠れてね。
Hey, hide while I'm counting.
Explanation
「〜ているあいだに」 means 'while' or 'at some point during'. The first part is a lasting action or state (sleeping, being out, waiting), and the second is a single event that happens at some moment within it: 「寝ているあいだに、地震があった」 (An earthquake happened while I was asleep), 「留守にしているあいだに、泥棒が入った」 (A burglar broke in while we were away).
Mixed up with
「あいだ」 and 「あいだに」 differ. Without 「に」 both actions last the whole time: 「子供が寝ているあいだ、ずっと本を読んでいた」 (I kept reading the whole time the child slept). With 「に」 the second part is a one-time event inside that period.
Unlike 「うちに」, which means 'hurry and do it before the state changes', 「あいだに」 is just a plain stretch of time.