The center of discourse and industrial endorsements, the good old Food Pyramid created by the FDA in 1984 (now re-created), can be used effectively with children. Because kids don't think abstractly until about age 11, a visual guide is often useful to illustrate a point.
The old pyramid had its deficiencies (check Harvard University's approach to old and new) but there is an interactive exercise that I have tried singularly with a 5, 6, and 9 year old. Each had questions about balance in diet, which led to questions and considerations of balance in day-to-day living. I was impressed, being a detractor of the old pyramid and not head over heals with the newest (here's the kid's page). This useful tool can help a child learn how to eat a balanced diet. It's up to you to put the balance in "balance," though.
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