Motivation and involvement are the keys. They need a purpose to write and we must offer them an amazing and an amusing prompt to develop this basic language skill.
A year ago, in a British Council Seminar in Barcelona, I discovered a web site that allow children to design their own Super Hero, with hundreds of possibilities (different parts of the body, clothes, weapons, mascots, colours...)
In the following PowerPoint you will have an idea of the process, as we were taking some photos and recording our “steps” towards the final goal.
This is Oumaima's Super Heroine...
Or Rafa's Super Hero...
Of course, there are many other possibilities using this type of “prompt” for writing (or even talking!). If you have new ideas, please share!