Writing is an important part of learning. Kids need to learn how to write, and how to write well for multiple reasons. Some of those reasons include to expand their imagination, problem solve, determine what will happen next in a given situation, to explain what they have learned, and to record their own feelings and emotions.
There are 4 main styles of writing. Each one has it's own special format and it's own purpose. Expository writing is subject oriented. The author is trying to tell you about a topic or a subject. The writing should contain facts and shouldn't have opinions. This form of writing is usually written in a logical order. Some examples of Expository writing are text books, reports about animals, countries, and science reports.
The second is Descriptive writing. This is where the author focuses on describing a character or an event in great detail. Many adjectives should be used in this form of writing. The writing should create an image in the readers head. It should describe places, events, situation, location, ect. in a highly detailed manner.
Persuasive writing should contain opinions, facts, and attempt to persuade the reader to see, and even agree with the author's point of view. It needs to have reasons, arguments, and justification behind both. Many times this form of writing calls for the author to take action of some sort.
Finally we have Narrative writing. Narrative writing is most often seen in short stories, novels, biographies. The author places himself in the story as a character and narrates what is happening throughout the story. This form of writing contains dialog between characters, follows a logical sequence. It also generally contains a situation or a problem or a dispute of some kind that needs to be solved.
There are other sub-forms of writing but most of those will fall into one of these categories. It's important for kids to understand how to use each of these styles so they not only can write to many audiences but they can determine which style they enjoy using best.
Here are a couple of sites I have found that contain writing prompts to help kids get started with the writing process.
Writing Prompts.net - This site has tons of prompts from grades 1-8. They also have specific prompts for each of the writing styles. (Creative writing on this site is what I would consider descriptive writing). I love that they have 50 ideas per grade and 100 prompts per writing style.
Journaling Helps has a list of 52 writing prompts for elementary kids. This site also has ideas and tips about writing journals and the different types of journals that you can keep like a nature journal, personal journal, Classroom journals.
To get kids interested in writing the best thing you can do is have them write something everyday. They don't always need to go through the writing process (draft, revise edit, publish). They don't always need to have a specific topic or do tons of research. While learning those skills are important I wouldn't do it every day. That can make writing seem like a long difficult task and can discourage kids from enjoying writing. I do believe they should go through the writing process once to twice a month. For many writing is the way we process the world around us and can help us see an insight into ourselves as well as others.