The Body...in the DIfferent Seasons collection...plus, you can view the movie, Stand By Me...to get an idea of what is inside. When I was a kid, after hearing my Aunt Be a talk about 2001, A Space Odyssey, I wanted and did read the book. The Body is about 11-year-old boys...Bea...only Wally could have an Aunt Be.
Yeah, the body is about young boys, and a somewhat wholesome story, but it does have a sex scene quite early on from the other story by adult Gordie.
Tom Gordon is a good choice, but again, it does contain a (admitedly brief) mention of Trisha's astranged parents having sex, and has some choice language aswell.
I think I can recall reading somewhere that Stephen wrote Eyes of the Dragon for his own kids, so maybe that is the best place to start.
Of course, alot depends on what level you think your child is at, and whether or not you are ok with them being exposed to sex and bad language, and sometimes violent themes. If he/she reads Eyes of the Dragon and likes it, they will possibly want to continue reading King, and by the time they are 12 or 13, your child will be left with only the "bad stuff". So as long as you think your child is level headed enough, and is aware of the nature of the books and their themes, then start with Eyes of the dragon, then probably Tom Gordon.