My parents often tell me I shouldn't be taking my kiddo grocery shopping. Go after he goes to bed, they say. When they watch him, I can go, they say. I've resisted.
Now, don't get me wrong, he's not a perfect angel every time, but I've been taking my son with me to the grocery store nearly every week since he could sit up in the cart (around 6 months). He holds the food, he picks out a fruit or veggie from time to time. (He's only 20 months, so he doesn't really get it, but that time is coming.)
This way, he doesn't think that the pantry and fridge make the food. He has some idea of our starting place. And someday, we'll plant a garden in the backyard, and he'll get an even deeper understanding.