Most are not new and should be available through the library. Once to you have expanded your Writer's Toolkit, you will be in a better position to help your kids. Try the different successful approaches that you read about. Let trial and error work for you and your teenagers. Finally, allow your kids time to find their own voices, using processes that work for them.
Tiny volume is a powerhouse of grammar and usage.
Read essays by eleven homeschooled teenagers, published authors all. Practical suggestions for helping writers aged 10-15 with the writing process. One of my favorites. Intended for adults but so well written, most teenagers will enjoy it as well. Great advice on everything from researching to generating exciting opening paragraphs.