Depending on where your dc is at already, Writing and Rhetoric would be another choice. We are doing MCT this year, but using Writing and Rhetoric for writing. The suggested grade levels would be high for a regular student. My dd should have been in 3rd last year by age, and we did the Fable level. She liked it, wrote willingly, and learned things. A 10 y.o. student might start in Narrative.
http://classicalacademicpress.com/writing-rhetoric-1/If dc is already writing well, consider Lively Art of Writing (there a free open source workbook) with some essay topic changes- those in the book are a little dated- and a subscription to Stone Soup Magazine.
HTH Bean
