Have you ever dreamed of becoming a mermaid (when you were a child)?
Have you ever dreamed of becoming a mermaid (when you were a child)?
(子供の頃に)人魚になりたいと思ったことはありませんか?
Have you ever dreamed of becoming a ~?
~になりたいと思ったことはありませんか?
子供のころになりたかったものの話をする時によく使うフレーズです。
"when you were a child" はなくても雰囲気で通じます。