How many times each day does your child try to talk you into something? Whether it is to let them go somewhere, do something, have something, or watch something, children are always trying to persuade parents into something. Use that as the wonderful teaching opportunity that it is!
Set up a precedent in your family that when your child wants something he has to submit it in writing. Once your child submits his written request for whatever it may be that he wants from you, sit down and read the request, then decide whether to grant the request based on parental discretion and how persuasive the letter is.
In order to begin this in your home, you must first teach the necessary writing skills by going through the first written request together. The necessary components of a persuasive paper include: a description of the desired item or action, a reason why you should let him have or do what he wants, and a preemptive argument to satisfy any questions that you would raise objecting to the request.
For example: say that daughter Sally wants a cell phone. First she needs to describe what she wants - type of phone, number of minutes, etc. Secondly she needs to explain why she wants it. She needs to give her argument. Finally Sally needs to think about questions that mom would have and answer them. This could include who is going to pay for it, how many minutes she would be allowed to have, etc.
The more persuasive Sally is, the more likely mom is to say that it is a good idea to get Sally a cell phone. At the same time, parental discretion must play into the decision regarding the cell phone. If the family is living on a tight budget and a cell phone is not financially feasible, then regardless of the persuasiveness of the essay, Sally still will not get the cell phone.
Whether or not you allow the persuasive writing to actually persuade you to allow your children to have or do what they want is irrelevant. The important thing is that you are getting them to write and getting them to practice the specific skill of writing persuasively.
Here is the writing process to help you show your child how to start and end a paper.
Pre-write: The pre-write involves getting those ideas flowing. Use an outline, free flowing ideas on paper, or any other method you can think of to organize ideas.
Rough Draft: After you have ideas roughed out on paper, put them in paragraph form. Do not worry about spelling, punctuation, grammar, or fixing any mistakes right now.
Editing/Revising: Editing involves fixing errors in spelling, capitalization, punctuation, and grammar. Revising involves changing words, moving sentences/paragraphs around, deleting and adding details in order to better the paper, make it more understandable and more focused towards the target audience (the target audience is that person or people whom the writer intended to have read the paper).
Final Draft: Once the editing and revising have been completed, the final draft is written. It should be typed or written in your child's best handwriting. The paper is finished in this stage.
For more information on teaching writing at home, and on Writing Disabilities.