In conversation, step by step, not too direct. But not being direct doesn't mean not making the problem clear. If you don't speak, you have to speak thoroughly.
Pay attention to your attitude when you speak. Don't think of yourself as an uncle or elder. It will be better for the other person to think of himself as a friend. You can't treat 13 years old children as children. Thirteen years old!
When you speak, you should focus on telling or hinting at each other. Only you know him and don't want to tell anyone. Also, communicate with your children more, and don't wait until something goes wrong before you regret it!