Instructables to D-I-Y manuals, humankind is involved with imparting knowledge. Kids are no different. Becoming an expert on a subject is also a way to involve reluctant readers - read more on this at TWRCtank.com.
One way to get your kids expressing themselves, especially in writing, is to ask them to explain how to do something to others. In writing, this type of text is known as a procedure. Procedures explain how to do something and we see them everywhere. Recipes are one kind of procedure; an instruction booklet for building a LEGO model is another. Pointing out these types of materials is a good way to bring them to kids' attentions. See what children can discover about procedures - can they see lots of verbs, are there numbers or some other way of showing order, do diagrams or photos accompany the text? One blog I love where artist Scott Bedford shares instructions for great projects is What I Made, and They Draw and Cook has illustrated (hand-drawn) recipes.
Your child could write a simple procedural list, then go on to make a video. Or take photographs to help his explanation, and annotate them on paper or in a slideshow. No matter the media he chooses, there's lots of learning involved.
Before we start writing a procedure, it's best to think about it first. What exactly do you want to share? Say it's how to play a favourite board game. That's your goal, or purpose, and often it becomes your heading. List all the equipment or materials you need, then write the instructions as simply as possible. Keep to the order the steps need to be in, if necessary doing each step and then recording what you did by jotting it down. Ask someone to read your procedure to see if it makes sense. Has anything important been left out? Would diagrams or photos help a reader understand better?
There are websites that facilitate publishing a procedure digitally so we can communicate with a wider audience. Slideshow sites like Slideshare and PhotoPeach are two. Kids could also write a procedure and publish it at Glogster or Notaland. Software like PowerPoint and Keynote is another format to use. One recent find of mine is Tildee, where people create their own "how to do" pages. Here's an example of a tutorial at Tildee on how to make a handmade clothesline bunting card.
If you're interested in children's writing, you'll find more articles under the Writing label in my right sidebar. One of my most popular posts is How Do Kids Write a Book Review?