I think all sports are inherently risky, but there are certainly some that are safer than others. Those that work with gravity rather than against it such as swimming are probably the best.
Safest? Maybe Tennis, Bowling, or Golf. But (assuming it's a boy) I don't think that would be overly much fun for a kid. I was always big on karate. It's usually pretty safe if you have good instructors, fun, alot of physical work, and gives them a skill they could need in the future.
I think tennis is a fairly physical game that keeps kids from getting banged up too. They have to be quick and develop a strategy and skills of their own at the same time. Plus it really requires a lot of coordination.
I think there is a chance of kids getting hurt no matter what sport they decide to play. You just need to let them play because you cant protect them from everything.
What if they hate the sport that you think is safe and decide that they can play?
I would think something like raquetball, or that one where you whack the ball that is tied to a string on a pole. Cant remember the name but the worst that can happen is you get hit in the eye by a wiffle ball.
--------------------------------------------------------------
Finding my muse