I have a six year-old son, so I get questions like this asked of me all the time. Thank goodness, then, that there are people like Randall Munroe of XKCD to answer it. Here’s what, among other things, would happen if a pitcher could throw a baseball at the speed of light:
These gamma rays and debris expand outward in a bubble centered on the pitcher’s mound. They start to tear apart the molecules in the air, ripping the electrons from the nuclei and turning the air in the stadium into an expanding bubble of incandescent plasma. The wall of this bubble approaches the batter at about the speed of light—only slightly ahead of the ball itself.
Which is why Greg Maddux chose to only hit the low 90s with his fastball back in the day.