Here are a set of websites for teaching children to write.  I also think they are valuable for adults wanting to write children's books.

This site has many really nice resources:

http://www.teachingideas.co.uk/english/contents05writingfiction.htm

This one is a course to show how to teach children:

http://www.suite101.com/course.cfm/17315/seminar

I have more, but what I suggest is to do what I did: put How to Teach Children to Write into a Google search box and see what comes up.  There's lots of good information out there!