Fetching details from the dictionary...

young; innocent; childlike
幼けない子供たちが庭で遊んでいた。
Young children were playing in the garden.
庭で遊ぶ子供たちは幼けなかった。
The children playing in the garden were young.
声は高いがあの子は幼けなくない。
The voice is high-pitched, but that child is not as young as it seems.