ritardando

One of the most difficult things that I teach little kids in piano is the concept of "ritardando." A *gradual* slowing down of the music. They just don't get it. I explain, with huge exaggeration, the "gradual" part, and then when they go to play it, they come to the "rit" and suddenly play everything twice as slow. What do I need to do? Count out loud and wave my arms to indicate the gradually slowing notes like my college professor?!