There is a lot of nutrition information out there, and while you may be eager to set your child up for healthy lifestyle choices, it is important to understand the age appropriateness of certain teachings.
Specifically, children ages three to five years typically learn best with hands on tasks that involve tearing, mashing and washing. They will not be extensively using utensils, but they will still be practicing some motor skills. They will learn most about nutrition through observance of the other tasks going on around them.
Kids ages five to seven years will have much improved motor skills, and safe and appropriate cooking utensils can be used. There are kid friendly products such as knives, graters and mixing sets. Think ahead and prepare for additional tasks that can easily be incorporated with just a bit of prep (e.g., have your child crack the egg in a small separate dish and then add it to the other ingredients). Additionally, at this age, kids can begin incorporating newly acquired school-based skills in the kitchen. For example, a recipe may be practice for reading.
Once your child becomes a teen, they should be able to participate in almost any part of meal preparation. This may also be a great time to begin introduction of more formal nutrition education. Eatright.org offers lessons on Teaching Your Teen about Nutrition Facts Panels. This is a unique opportunity for you to help your kids forgo many of the nutrition myths out there.