You can show your children that carrots actually look like this
Your kids can learn that potatoes grow underground and that tomatoes grow on a vine.
You can do all sort of math projects, like measuring your plot, weighing your production, counting seeds and seedlings. And the science: Everything from labeling plant parts, the biology of reproduction, to measuring the pH of your soil!
Entomology---good bugs and bad bugs in the garden
The vocabulary: decomposition, organic, pollination, germination, etc.
Geography, economics, nutrition.
And the greatest for me: the scriptural application and Truths found in dirt, seeds, plants, vines, water, giving, receiving...well, you get it.
And then there's just the good old hard work, not to mention the yummy good taste and high nutritional value of fresh vegetables.
So? What are you waiting for? Think it's time to start growing some veggies of your own? Experience is the best teacher of all!