My nephew knocked a kid out with a pitch to the helmet once. He was a great pitcher, struck almost every batter out, but one pitch slipped out of his hand and the kid went down. They were only like 9 and 10 years old, but he threw hard enough to bring a kid down. He was OK luckily because of the batting helmet but a pitcher is pretty vulnerable on line drives.
Don't worry too much, it really is a fluke when it happens (someone gets hit by a line drive). The kids will most likely be fine, so don't hold them back from playing out of fear. I played and was hit by a lot of wild pitches and I survived.
Deaths are really rare, mainly out of luck I'd guess though.