Young Athletes obviously need to be Physically fit to Sustain the Demands of Games. We use Fitness as a General Term. However, we Systematically train and prepare in this manner:
1- Strengthening (to get the muscles strong)
2- Conditioning (Implement Soccer Specific Training)
3- Agility + Balance (Add a Soccer Ball, for instance, to encourage game like balance)
4- Endurance (Building Stamina) 
With such an Approach, Hundreds of our Players have Enjoyed Successful Seasons, Injury-Free!