ruby - or Java?

Decisions, decisions. I'm trying to write a small program to help my son learn to do subtraction. (His problem today is borrowing, which is these days called "regrouping.") It is a trivial sort of GUI program and I'm trying to decide whether to learn Java UI programming with Swing, or Ruby/Tk. Swing has the advantage of including anything I could ever possibly need and working out of box. Ruby/Tk isn't nearly as all-encompassing ... but ... let's face it: Ruby is 10x the language that Java will ever be. (Hence groovy.)