Adding to Natalie's post...
I can recommend The Art Institute of Chicago as an online resource for lesson plans for young people. Please see: http://www.artic.edu/aic/education/trc/lessonplans.html
These resources obviously are targeted to promote the museum's collection, rather than being basic design information, but they are pretty good.
I've come across a series by Dorling Kindersley called The DK Art School, which has introductory titles on art technique, perspective, figure, drawing, oils, water color, pastels and various other media, with alot of information on technique & copious examples and illustrations. I'd say this series is appropriate for upper elementary students on up.
DK also has an Eyewitness Art series that has titles on composition, perspective, color, looking at paintings, as well as titles on art movements like post-impressionism and also on individual artists. I think these books would also be good for 5th-6th graders and older.
Finally the Usborne Book of Art by Rosie Dickens is a great general overview book, with an introductory section on how to look at art, followed by chapters on various historical periods in art, and ending up with a chapter on artists' materials, tricks of the trade, and even forgery! There's a great timeline on art history and some brief biographical blurbs on various artists, plus a glossary of art terms. Throughout the book are sprinkled internet links that take readers to Usborne sites with even more information and activities.
Here's the full list of links to Usborne sites: http://www.usborne-quicklinks.com/usa/usa_full_book_list.asp
While the book is targeted to upper elementary and older students, the internet links offer activities for multiple ages.