Quick take: Development of youth pitching.

The development of youth pitching is one if not THE most difficult assignment at this level of coaching. You don't want your young pitchers to be throwing stuff that they physically aren't ready for yet. Stick to your four and two. Stay away from curves if you can and especially from your breaking ball that twists the arm. 

Each player developes differently from bothe a skill, and physical level. Understand your age group, and that your athletes bones are still growing, and with that will come limitations on movements that they should be repeating .