A great resource for writing is the "Everything You Need to Know about English Homework" book. It covers a number of different aspects of English, from word ederivations to parts of speech to how to write a business letter. I particularly like the information about how to write a story, and different methods of outlining.
I have found that when I encourage my kids to write about something that interests them, they can't seem to write enough about it. They love it! When I suggest a topic they aren't necessarily interested in, tHen it becomes a chore, rather than fun and interesting. I think it's definitely better to let the kids write about what interests them. In real life, I don't want them to take a job they hate - I want them to know what interests them, and to pursue their own goals and interests. I think schooling should lead them toward that. Rather than giving kids the idea that learning is a chore and work is boring, but you do it just to please others, I'd much prefer my kids know what they like, love pursuing their own interests and love learning about things that interest them.
Of course, the more children write, the better they are at it. Here are some websites from some kids that I thought were cute:
Young Volunteer
Truth About Drugs
Building a Better World