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.
先生の前でいい子になるのはやめなさい。
Stop acting like a good kid in front of the teacher.
彼は会議でいい子に
He played the good guy at the meeting and took all the credit for himself.