act like a child

To behave in an immature, petulant, and self-centered manner. You're acting like a child, Sarah! This is just the way the world works—it does not revolve around you! He's a spoiled, entitled jerk who acts like a child when he gets bad news. It's no surprise he threw a tantrum when they said they weren't renewing his contract. Would you two please stop carrying on in the middle of class? You're acting like children!