These are a summation of some of the 'rules' we talk about in our home (we have five children ages any-minute-now newborn to just-turned seven).
1) Only eat food. Many non-foods are cleverly disguised as food, but don't be fooled; they are not food. Don't eat them.
2) Only eat what you can/would prepare in your own kitchen. I couldn't/wouldn't make sodium benzoate, fd&c yellow #5, partially hydrogenated vegetable oil, or maltodextrin (amongst myriad, seemingly nearly infinite 'additives') in my kitchen, so I don't eat them. This correlates closely with number one, but warrants special mention because of the unbiquitousness of these additives that are not a part of the title of dishes commonly served like 'chicken soup', which is a food, but only if it's really chicken soup and not processed with fifteen additives including hyper-refined salts, sugars and grains- also cannot/would not make in my kitchen.
3) Eat from local, synthetics/toxin-free sources as much as possible. Though being proponents of traditional foods cooking/eating and being first generation citizens of our country, we find this very challenging. We are not living in the climate that produces the foods that our bodies feel best consuming. So we do buy and eat things like organic olives, unrefined organic coconut oil, and a few other items that cannot be grown here. We hope to move back to the much warmer climate our parents (and theirs and on and on) came from in the next five yrs though. Can't wait!!!
4) (This is my seven yr old's solution to figurng out if something is truly food, so I'll include it because while it's not fool-proof, it's a pretty good guiding idea) Don't buy anything with a list of 'ingredients.' Preferably grow, but otherwise buy your own ingredients and make food!
And of course this all rests on the rule that probably should be first:
5) Learn how to prepare and use all of the edible parts of any source of food; it should be tasty, nutritious and respectful of the source and the diner. This is the best preparation of a food and a dish that takes a week (or more) in prep. before it is eaten is worth the wait!
Right now we are having a hard time getting our boys to eat at the table for any meals because they are foraging all day outside! So, many of the food rules we are talking about now are about how to decipher edibles from inedibles that grow wild on the property and also when each vegetable is ready for harvest (and please don't pick off teh flowers or pull the plants while they are still babies!).

I do love the green, yellow, and dirt-coloured stains that they have on their faces all day from eating so much stuff out there though. They feel so empowered to feed themselves; it's wonderful!