Is your kindergartener reading yet? If so, have him copy a few lines from the beginning readers. The words are failry simple, and they can actually read what they're writing. Also with younger children (even Faythe at 12!) I have found it works better to type what it is you want to copy on the computer in a BIGGER FONT, with 1 1/2 spacing. That way they can follow along better. I do a lot of my copywork from history or Bible verses from AWANAS. It works better for me if I print a whole week's worth on Sunday, so I'm not scanning books at the last minute for something!
It's too easy to get copywork for me to be willing to spend money on a book.