This is one of the most common questions you will hear from young pitchers. I am not going to get into much of a debate about it, but what is clear is that kids should first learn how to throw their fastball with intent and learn to place it where they want. Then they should develop a secondary pitch. This pitch should focus on developing a pitch that disrupts the timing of the hitter. Hitting is about timing, our job as a pitcher is to disrupt that timing. That is why the changeup is such an important pitch for kids to develop. Check out this video (with great graphics) with Bill Ripken as he explains the fastball changeup combination and why it is so effective.
Once they have developed this combination and they their body has begun to mature they should begin experimenting with grips and angles to feel what the ball will do.