To bring up something that is best left alone and thereby cause unnecessary trouble or provoke a dormant issue (literally 'to wake a sleeping child'). An idiomatic expression.
あの話はもう終わったことだから、今さら蒸し返すと寝た子を起こすだけだ。
That matter is already settled; bringing it up again now will only wake a sleeping child (needlessly stir things up).
会議で問題点を指摘するのは重要だが、今は時期が悪くて寝た子を起こすかもしれない。
It's important to point out issues in meetings, but now might be a bad time—you might be stirring up trouble.
余計な言葉で寝た子を起こすことのないよう、発言には注意した。
I was careful with my remarks so as not to wake a sleeping child with unnecessary words.