Hi Willa,
My two daughters took CTY distance learning writing classes and they loved them. The classes are mostly for middle and high school kids (5th grade and above). Your kid might be a bit young for that. The kids also have to take CTY tests to get into the program first. The cost is about $800 per class last about 3-4 months. For more information:
http://cty.jhu.edu/writing/courses.html Another option is to find a mentor who can do e-mail exchange with your kid. Give comments, correct grammar errors, etc. If you are interested, send me an e-mail clu@cisco.com and we can talk off-line. I may be able to get my younger daughter Yanran (13, 9th grade, YS) to do that.