This is a kind of open-ended question, but I'll try!
If you've used Swing, you can show them the ease and comparatively shorter code to implement similar apps. Plus, JavaFX gives you the graphical/animation/2D stuff as well.
For example, the following example
Launch Order Your Pizza uses Swing-based components from JavaFX. It also uses graphics to enhance the application. I haven't tried to implement it in Java/Swing, but I know it would be longer, even removing the cool graphical elements.
If you already do
Java, you have the full Java API.
JavaFX is easy to learn. The declarative style makes code easy to read and shorter. It is statically typed (JavaScript programmers will appreciate this) and provides a mature architecture for building robust applications. (Again, JavaScript/Ajax programmers will appreciate).
NetBeans provides a lot of help for learning. There is an Eclipse plug in too.
It's free.
It runs on the JVM.
And our book shows you how to write cool JavaFX apps