How do I get a refund for a purchase I didn't intend to make?

My son is on family sharing. I have it ask me for permission to purchase anything. During the last update, I think it changed that setting. It has always asked me for purchases. Or it could have happened when he got a new phone. Didn’t know I should have checked. But he says he accidentally purchased something and he couldn’t stop it. But it doesn’t show up on my purchase history. How do I get a refund for this purchase?

[Re-Titled by Moderator]