Those books are great ideas too ! 

That also made me remember there are some pretty good kids videos about farm stuff too.

Chris Fesko has a whole series of Videos here:

http://www.fesko.com/order.html

Another really good one is "Tractors Combines and Things on the Grow"

http://www.cottonsjourney.com/productcart/pc/viewPrd.asp?idcategory=4&idproduct=52

A short video clip of it is here:

http://extension.usu.edu/aitc/teachers/movies/tractors_combines.html

Another one by the same folks is "Tractors Cotton Pickers and the stuff kids Wear". Both are very good at illustrating where food and clothing comes from in an informative way for young children. Even if they don't understand the words they will want to watch it again and again.

These were produced in 94 and 95 by just our size videos on VHS tape. Not sure if they are still being produced or are available on DVD or not. Only contact info. on the video sleeve is 1-800-808-FARM

Both of these are about 30 minutes long