I don't think I could give a 101 on autism and I've been living it for 5 years. The main thing I would tell your visiting child is to be polite. Just like it is not polite to make comments about obvious physical disabilities or characteristics, no one should make comments about an autistic person in front of her/him like they don't hear you. Example: At a birthday party for a 5yo autistic boy who could not yet manage a fork to eat his cake, a friend of the sibling exclaimed, "Why is he eating like a pig?" Other don'ts include, "Why is s/he screaming?" "Why is s/he jumping around so much?" "Why is s/he twirling that toy constantly?" etc, etc. Your child should talk and attempt regular interactions, but not get feelings hurt if the attempts are not reciprocated. I wouldn't try to explain autism exactly to your child, but just the usual stuff of respecting differences in people. Hope that helps a little! Nan