Fastballs are the most dangerous pitch on the arm. The fastball has been to show having the greatest stress on both the shoulder and elbow. When you couple that fact with kids playing more baseball than they should, you have what is called "overuse". Overuse is the number one leading cause for pitching/throwing injuries. Studies have shown that pitchers who throw breaking balls (including sliders) are no more at risk than those who don't. Again- Ask yourself these questions on the health of your kids arm-
Does my kid pitch while injured or fatigued?
Does my kid play baseball more than 8 months out of the year?
Does my kid play multiple position that require a lot of throwing after he pitches?
Does my kid throw over 80 mph?
Answering yes to any of these questions places your kid at an increased risk for injury (yes, even just throwing hard increases risk). This means you should be aware of the main risk factors and learn to stop, limit, or control those situations. No one should be pitch9ing more than 8 months out of the year. No one should pitch while fatigued or injured. Pitchers should rest after pitching. If your kid throws at 80 mph or above he really needs to learn to stretch properly and condition his muscles and ligaments to prevent injury.