I'd have to go with corn snakes. They stay relatively small (only about 5 feet), they are very docile, and very easy to take care of. My first snake was a corn snake actually.
A lot of people will probably recommend a ball python, but I wouldn't. Although ball pythons are extremely easy to take care of, they are also very picky eaters. They will randomly refuse food for weeks or months at a time for no apparrent reason. Ball pythons are good snakes once you have a little experience first.
Another reason I'm going to recommend a corn snake is they are very inexpensive and have a variety of color morphs that are easily obtained. You can get pretty much any color morph of the corn snake for under $100 (most being in the $15 - $50 range).
I've never been bitten by a corn snake, and I've never had a corn snake refuse a meal. In my opinion, a corn snake would be the best way to go if you are getting it for a kid.