You first have to determine the purpose of the letter before writing. There is a reason why you want to write a letter. You might be writing to the city mayor, for instance, to complain about the state of public schools or play parks in your area. Or you want to thank your governor, or senators for a new legislation. You can write for any reason.
The format of a formal letter for kids is just the same as that of a regular business letter. It starts with a heading--your address in the top right corner of the letter. Skip two lines and below put the date on the left side of the letter. Skip another two lines and write the name and address of the person or organization you're writing to. The addressee must be addressed formally when you put the name on top of his address -- "Mr. Goodwell Jones."
You have to jump another two lines after the contact information of the addressee and greet him "Dear Mr. Jones:" Skip two lines before starting to write the main body of the letter. The first paragraph should explain why you're writing the letter.
Expand your opinion or request in the next paragraph, using facts and details about your concerns. It is important to learn about the subject before writing your letter because you have to back your opinion with facts to sound informed. Propose the action you would like the address to take before thanking the person for reading your letter. End with "Yours Sincerely," or "Sincerely," and sign the letter.