What you actually get to decide

You decide what you are willing to fund, and you are entitled to that without apology. You do not decide what they study. Confusing the two — funding made conditional on a subject — buys you a compliant eighteen-year-old and a resentful twenty-five-year-old, and the ones who switch back later have usually lost more time than the original choice would have cost.

The distinction is worth stating plainly out loud, because it lowers the temperature immediately: "This is my decision, that is yours." It also makes clear that you are not withholding support to win an argument, which is the fear underneath most of these conversations.