I teach 3 year olds part time. Last week was our farm unit. Most of the week is spent talking about farm animals; we sing animal songs, play animal games, make animal noises, pattern with animals, work with animal related manipulative items - even 'milk' a cow (our easel transformed into a cow with a rubber glove filled with water as the udders).
We do however throw in a little talk about food grown on farms. We read books about plants needing water, soil and sunlight to grow. We pretend to be farmers and pretend to plant, water, etc. Our science experiment for the week is to take a leafy stalk of celery and place it in water with red food coloring. By the end of the day, the stalk and leaves have a tinge of red to them and the kids can see how a plant needs water.
I've done this lesson in the past and didn't even give it much thought. This year, it really struck a chord. The red dye is was in the water, the plant 'ingested' that dye. I am sure there is a lot of higher level science involved = size of the particulates, temperature and evaporation rates, blah blah blah. But the basic, preschool level thought of the day...
If chemicals are sprayed or added to the soil - The plant will 'ingest' it.
Maybe we should add funky colored dyes to any chemical put on or around conventionally grown plants so people can SEE that those things are actually there in the 'final product'. Oh wait - that would just be marketed as fun, exciting, and would have a character endorsement.