Emphasis should be on multilateral development (playing different sports) in order to build:
Athleticism
Social skill and collaboration skill (adapt to different teams and environments)
Prevent Burnout: Early specialization can lead to fatigue and loss of interest (Gould & Udry, 1994).
Prevent Overuse Injuries: Repeatedly engaging in the same sport can increase the risk of overuse injuries (Jayanthi et al., 2013).
Expand Motor Skill Library: Playing multiple sports helps develop a broader range of motor skills (e.g., coordination, balance) (Balyi & Hamilton, 2004).
The content should consist of approximately 70% fun and 30% structured training. The focus should be on minimizing injury, while also building enjoyment and passion for the game (Balyi & Hamilton, 2004).
Reference: https://www.sportsmith.co/articles/developing-a-youth-athlete-development-roadmap/