Go with why_wolf's suggestion but set up a simple demostration within the classroom.
Hang up a curtain, roll in a blackboard, or otherwise have two volunteer students separated back to back with individual chessboards and pretend telephones.
Kids love hands on and learn well accordingly. If you have the resources - use two cell phones instead of pretend phones.
Then challenge the kids (create teams) to come up with a simple way to communicate just the first simple move (a pawn or knight) using only sound or light. No verbal communications. They will quickly see what it really takes to process even something simple......