From the Home School Legal Defense Association.
While all Christians should support people with disabilities, this United Nations treaty includes language that could one day be used to trump U.S. law and prevent parents from caring for their disabled children as they see fit. Besides, the treaty is unnecessary, since U.S. law is already the best in the world at protecting the rights of persons with disabilities.
From the above link, and click on "Take Action." You will have to register your name, address and telephone number to route your letter to the right Senators. Follow the prompts provided on the HSLDA website.