I used to teach Shurley. I do not recommend it for only one or two students. For a classroom, it's great. It's fun, engaging, on topic and age appropriate, and it covers all the learning styles.
I am using a program (I just got home form a seminar on it) called Spell to Write and Read. While it is not specifically a creative writing/grammar program, there is a program that meshes very well with this at about the third grade and up called IEW (Institute for Excellence in Writing). Writing Strands is also good for middle school ages.
Clare likes Institute for Excellence in Writing. TRISMS has incorporated it into their curriculum It would be great for Erin to start this at the beginning of her middle school years. The whole IEW program, lessons and Seminar CDs costs about $150, but it would last her all the way through high school.