You'll need to determine, so that you can let them know, exactly how their job performance is evaluated. It won't hurt to exaggerate your own role in that evaluation.
Let them understand how the system is designed to provide feedback for good performance, as well as negative results for underperformance.
Tell them you are a neutral witness, and that it is entirely up to them.
Actually I have no (direct) input into their job performance rating. We all report to the same guy.