As a special education teacher, I guess I was under the false impression that my child would grow up automatically accepting other children with disabilities or differences. Sadly, I was very mistaken. I wanted to take this opportunity to encourage everyone to take time with your child discussing differences that other children might have. Assure them that they also have many things in common with children with disabilities, one being feelings. Everyone can feel acceptance and sadness...everyone has feelings.
My favorite poster to keep up in my classroom...

If you don't typically come into contact with children with disabilities, you might look at some websites with your children such as http://www.specialolympics.org/ or search for children's books about disabilities.
Remember that before you automatically tell your child to stop staring, that looking, learning, and being curious about differences is not only natural but a good thing. We just have to teach our children compassion and acceptance.
I'm sure that parents of children with disabilities get tons of questions, but the main one I get as a teacher is, "Does he/she understand what's going on?" The answer is, "Yes!"
True story...
A boy with severe disabilities, not able to talk or walk, scoots over to watch another child play. The typically developing child yells out, "Ooo, I don't want him over here!" The boy with disabilities lowers his head, scoots away, and doesn't try to interact with another child all day. Another teacher says to me, "Wow, it's almost like he knew 'Joe' was scared of him. Do you think it really hurt his feelings?" I was almost speechless. Of course, it hurt his feelings! Everyone has feelings!
I know it won't be easy, but try to make a great Teachable Moment out of this with your children, before any more precious children get hurt. Let's all try to see with our hearts instead of our eyes!