When a child recognizes the ball it coming at him, many times he instinctively wants to turn toward the ball and many times the he will put out his hand trying to stop the flight of the ball.
Also if the ball misses the batter many times he will hold the bat so the ball will hit it for a foul ball and strike.
Teach your kids to turn their back and bend down placing the bat on the ground. If they turn and bend the ball will have a smaller target and most likely end up hitting the fatty part of the back or buttocks. Yes, it will sting, but better that than getting hit in the chest, groin, face, hands, arms, knees or wrists.
Also placing the bat on the ground better assures the ball will not hit it if the batter doesn't get hit, its a ball.
A good drill is to throw baseball sized whiffle balls at your kids and have them practice turning, bending and placing the bat on the ground.
The more you practice this, the more normal it is for the kids to implement this during games.