Since they have minimal background, your best bet would be to start them off with the very basics. If you're set on them using java, codecademy has a beginner course. Codecademy also has a python course which is also a viable option (and better IMO).
Once they have the basics of Java down, have them look at the
wpilib screensteps. These will help them in making robot code in Java.
If you aren't set on Java, I recommend Python. There is full support for it in FRC, and the community is very helpful. You can read more about it
here.
Whichever way they choose, I'm sure they'll enjoy it.