Getting hit by a batted or thrown ball are the two leading causes of injury in Little League baseball, including painful bruises, eye injuries or even broken bones and concussions [source: Little League]. Because backyard baseball is usually played in a confined space, you need to choose bats and balls that are softer and don't travel as hard and fast as the regulation gear.
Wiffle balls and bats are the traditional choice for backyard baseball. Made of hard plastic, the balls can still be hit far, but a whack in the head with the bat would only leave a nasty welt, not cracked skull. Nerf also makes bats and balls coated with their trademark foamy material for softer impact.
If you have a large open backyard and the players are older and more experienced, you can use standard aluminum bats, but still only use "low-impact" or "reduced-impact" baseballs. These are softer baseballs that have proven to reduce injuries in T-ball and Little League play [source: USA Baseball].