You're trying to teach children about the physiological benefits of good nutrition.
So do exercises where good foods give you good abilities, and bad foods give you bad abilities.
For example:
Place a coin in the child's hand and feed him a stick of celery. When he open his hand, the coin has been bent from his superhuman strength. Here are some more ideas...
http://www.thetrickery.com/full/Magic-T ... __5874.htm
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Another one would be a mentalist act.
Carrots make your eyes so good that you can see through blindfolds.
Vegetables make your brain so smart that you can memorize the names of every child in the gym.
Put a small object under a cup. Pour some sugar on it, and it gets fatter and fatter.