a simple thing that you can do is talk to them about how they are anointed to sing. This drops their guard because love is in operation. Next thing you want to do is encourage them to grow by letting them know that you want them to know what you know. If you are a giving teacher who walks in love, they will respond. finally, if you can, let them sing along with the CD while you play along with it. Fade the CD and keep playing, this trains their ear for both key, pitch and your playing style. Finish with more praise of how awesome they did, but not enough to blow up an ego.
satan attacks in music. if there is a way for him to get into a church, he will, but love overcomes all things. Scripture tells us that Jesus was moved with compassion (which is love in action), therefore, you have to do the same thing. Instead of telling them what they can't become overnight, show them who they've always been since the beginning of creation, and that's Children of the Most High God who are fearfully and wonderfully made and that God their father wants to hear from them.
Stay blessed.