To increase velocity on young players simply play catch more. Not long toss just catch. This act allows young players to gradually develop lay down on the forearm as growth plates set, avoiding future risk of injury.
A growth In lay down of 10% or more to quickly will almost guarantee injury. 5% growth equals 27% chance of injury. Weighted ball programs must be monitored to manage this. Hire an unbiased professional.
Players will gain velocity naturally based on physical age develop not actual age.
Gain 1.5 mph per year of age and 1.2 mph per inch in growth.