Hi Melanie,
While my kids have not tried this themselves, here is the address I have for letters to Mickey Mouse:
Walt Disney World Info/Guest Letters/Letters to Mickey Mouse
P.O. Box 10040
Lake Buena Vista, FL 32830-0040
If you would like to write a letter to Mickey, and include a self addressed stamped envelope, I have heard Mickey will send a letter back.
Another popular choice is to request a fairy godmailer on one of the popular DIsney chat boards, like
www.disboards or
www.passporter. If you visit the site and post a topic that you are looking for someone to send a note to your children, you will find a number of Disney guests who love sprinkling pixie dust. Numerous Disney guests that are headed down to Disney World before you, are often happy to take your mailing address and send a letter or postcard to your children, while they are at Disney World. I have served as a fairy godmailer many times and love being able to bring some Disney fun to a fellow Disney fan.