Microlite Purest Essence", because its beautiful, and so straight forward. I'm currently in the process of drawing up a little dungeon map and writing out a few NPC's and story goals/plot lines for them. The idea is to get them in the dungeon as quickly as possible and fight some monsters.
I had read an article a little while ago about a person who's child had autism, and they had started playing some D&D, in the end it worked out quite well for the little guy.
Here's the article - "How Dungeons & Dragons saved my autistic son".
One of my nephews is autistic, from reading the article I see a lot of similarity's between him and the author's son.
I think they are going to enjoy it thoroughly. It will be of course theater of the mind. The thing I really dig about Microlite Purest Essence is its so easy to house rule.