I recommend doing this as early as possible in their inclusionary school life. I am a strong advocate of building a strong partnership with your child's teacher so that you are working together instead of against each other. A wonderful way to begin this partnership is to offer to do a disability awareness session with your child's classmates at the beginning of the school year. Start with a story. Especially if you are nervous, this can be a great way to help everyone relax.
Young kids relate well to the characters in stories.
Make it fun AND educational. This probably goes without saying, but it's important anyway. Keep your sense of humor. Remember the basics. However, they will eventually need to refer to their disability as well.