As a special needs teacher, I have often been asked if they should talk to their typically developing kids about children with special needs that they may see in the community or at school.  My answer is absolutely yes.  Your child will most likely stare and may look a little frightened if they see someone that does not look like them.  The worst response to this behavior in my opinion is, "It's not nice to stare."  Instead, show your child that you feel comfortable by pointing out how pretty the girl's shoes are or how he and the other boy have the same baseball hat.  Later, talk to your child about the boy or girl that you saw.  "Remember today when we were in the store and saw the little girl with the pretty shoes?  She was in a wheelchair because she needs help getting around..."  The most important thing is for your child to realize that children with special needs like the same things that s/he likes.  The more you can talk openly with your child, the more comfortable s/he will be with children who have special needs.  Typically developing children can learn just as much if not more from children with special needs!  And it's beautiful to watch happen!