The split-finger fastball is a much more advanced pitch than the other three fastballs. It varies slightly from the forkball in that it's thrown with more velocity and generally replaced it as part of a pitcher's repertoire in the 1980s and 1990s. It dives when it reaches the plate.
To throw a splitter, split the middle and index fingers and grip the ball along the widest point of the ball. Don't jam the ball past the midway point of your fingers, but the grip is firm. The thumb is along the seam of the bottom, on the back seam.
Children generally can't throw split-finger fastballs because their hands aren't big enough.
Your index and middle fingers should be placed on the outside of the horseshoe seam. The grip is firm. When throwing, throw the palm-side wrist of the throwing-hand directly at the target while keeping your index and middle fingers extended upward. Your wrist should remain stiff.