Kids that age already know this, they simply aren't aware that they know it. The music they listen to every day is rhythmically sophisticated. Put on some rap or some popular dance music, get them up dancing and then stop them and get them to analyze their body motions. Show them how their bodies are subdividing the beat into rhythmic bits and they'll get it. Then give them a score that lines out their dance rhythms and they'll see it as well.

Just an idea.

      - Mark