Oftentimes the color wheel is taught in a boring manner, or it is over-taught (easy to do as it really is a very simple concept.
I recently found a printable worksheet created by M.C. Gillis at AwesomeArtists.com that teaches the theory of the color wheel without either dumbing it down, or making it dull. It is perfect.
There are lots of other activities on the site that are perfect for teaching art to homeschoolers. Give it a try!
Awesome.