Motivation for children can be tricky to understand. In my club (WSV Elite SC), I do my best to screen for motivated soccer players and families. After studying developmental psychology, I realized that most screening processes are sufficient, but the degree to which a coach understands developmental and performance psychology determines the quality of the developmental experience.
Every human has to establish a framework of interpretation through which they engage the world. Additionally, every young soccer player’s "soccer world" needs framing. In order to create a frame, motivations must be present.
Emotions emerge within the entire framework of interpretation. Emotions begin the process of goal setting. As goals are set, actions determine progress. As actions are taken, emotions act as a navigation system. For example, positive emotions indicate that you are moving towards your goal. Negative emotions are more nuanced. Some negative emotions indicate the need for adjustment/reflection, some require a complete stop and some justify quitting. Simply put, emotions within this structure are a forward, stop and reverse system.
Motivation sets a target and determines where to aim. Motivation organizes perceptions, and focuses the things that facilitate progress toward towards a goal. When encountering situations, people or things that aide progress towards a goal, positive emotions are produced. When things impede a goal, negative emotions are produced.
In youth soccer, the best coaches should spend a significant amount of time with the youngest and most impressionable players. Doing a good job with young players requires a tremendous amount of skill and training. The more training and education coaches and parents have, the better they will be able to support the player through the developmental process.