Here is an excellent site where kids post articles: Its the National Geographic Kids Blog.
I am going to start preparing my daughter also to write, so one day soon, she will be able to post an amazing article here!
I would also encourage you to go through the other features of the site. For little kids, there are recipes and craft items and a lot of information on animals and their nature. Try to watch some of the videos as well (if your internet is up for it!)