For goodness sake! What kind of planet are we living on???? I'd be horrified if I had an 18 month old who couldn't identify fruit and veg. This is unimaginable to me. Mind you, I have tried to teach them from early on about good food and of course Thomas grows it in the garden so they know exactly where it comes from too!
I have to say that one of the major advantages of having a big family is not being able to afford ready meals so the kids get to see how to make food from the bottom up and of course it also forces the big ones into having their own cooking days which further advances this knowledge. Tell them to go cook a meal for ten from raw ingredients, they are completely undaunted.