The Progymnasmata - Resources for Classical Writing
Nota bene: You will not ruin your child if you don't use the progymnasmata! If you have the time and inclination, the progym may enhance your writing instruction. If you don't, "regular" writing instruction is perfectly adequate, and you will still be giving your child a classical education. Do not listen to those who feel the need to loudly proclaim the superiority of the progym or any other method or approach or curriculum if your children are learning.
Books:
On the Teaching of Speaking and Writing (Books One, Two, and Ten of the Institutio Oratoria - pre-rhetoric) - Quintilian
Teaching Materials:
Composition in the Classical Tradition - Frank D'Angelo
Ancient Rhetorics for Contemporary Students - Sharon Crowley
Classical Rhetoric for the Modern Student - Edward P.J. Corbett
Classical Composition from Jim Selby
Classical Writing series
Websites: