I needed a good laugh though.... I don't know maybe try some like windex cleaner, you could try polish if you run out of cleaners to try. Other then that, crts are cheap so use it as an excuse to get them to upgrade. If your parents are like mine they're still using something you built in the 90's.
They sell car polishes in any automotive store, I don't think it would end up with the greatest results, but maybe better then it is now.