When kids are small, parents are responsible for the food they eat. But at some point, you have to send them off to make their own choices, and hope you've done a good job preparing them. It is important to to teach kids WHY they need to eat healthy and make smart choices for themselves.
I really like this book by Dr. Sears, Eat Healthy, Feel Great. It does a great job of explaining green light, yellow light, and red light foods in a way that kids can understand.
The USDA also has a great food pyramid designed just for kids.
I don't think it is ever too early to teach kids about making wise food choices, and how the choices we make effect our health and body (good and bad). It is our responsibility as parents to make sure they don't head into adulthood with bad habits and lack of knowledge.