Good question. History lessons about grass seed kind of ring hollow for people. Few people seem to know or care what soybean oil is or that mayonnaise and tortilla chips are full of it.
I would probably start with some Michael Pollan platitudes--buy perishable things, no barcodes, plan family meals, etc. If they ask for meal ideas offer some tubers to displace the grain, fruit to displace the sugar, and unrefined fat to displace the oils.
Lots of: urban hiking, cycling, sprinting
Lots of: fresh meat, seafood, eggs, organs, tubers, starch fruits, vegetables, meat fat, dairy fat, oil fruits
Some: cured meat, dairy protein, sweet fruits, rice, pulses, tree nuts, oil seeds
Minimal: soy, refined proteins, sugar, liquid carbohydrate, grains, refined oils, peanuts