Where do you live? There are a lot of good sites that sell Chinese learning books in general: Chinasprout.com, Childbook.com, Asianparent.com... (search "workbook") There are lots of different workbooks, usually in textbook series, like Chinese Made Easy for Kids
http://www.childbook.com/Chinese-Made-Easy-for-Kids-Workbook-Simplified-L1-p/lcmeksw01.htm (look at the link below, they have Trad character edition too), Chinese Made Easy (which is for a bit older kids), My First Chinese Words,
My First Chinese Reader (same company as First Chinese Words but again for a bit older kids), Kuaile Hanyu.
http://www.childbook.com/SearchResults.asp?Search=workbook&Search.x=0&Search.y=0The Max and Mei books have a couple of workbooks:
http://shop.ppp.com.hk/category/activity-book They are meant to go along with the story and cd sets (click "box set + audio CD" in the Categories menu to the right)... We have the first one and it is nice and simple start.
Mei Mei Hu of the Mei Mei dvd fame has some "Let's Write Chinese with Mei Mei" books:
http://www.meimeiandme.com/book01.html though she only ships within the states. She has other character writing and practice books and pads for sale as well:
http://www.meimeiandme.com/books.htmlHave fun! I'm only starting the most rudimentary with my son who is 5.5 yrs old... ie da, ri, yue, xiao, yi, er, san....