いい子こになる

expression, godan verb (-ru)

act like a good kid; be on one's best behavior; take all the credit

Often used sarcastically or critically when someone tries to look good or curry favor, sometimes at others' expense. The literal sense 'to turn out well (of a child)' is less common and usually clear from context.

• 先せん生せいの前まえでいい子こになるのはやめなさい。

  Stop acting like a good kid in front of the teacher.

• 彼かれは会かい議ぎでいい子こになって、全ぜん部ぶ自じ分ぶんの手て柄がらにした。

  He played the good guy at the meeting and took all the credit for himself.