Bottom line, to make your child a better long-term athlete, go back to the basics; movement of any kind, albeit different sports, chores or free-play will all help to create a strong base for an athlete.
Practicing things like yoga, tai chi or meditation for even 10 minutes can have a profound effect. There are many types of mindful activities. Find one that resonates with you, and immerse yourself in the moment.
Simple things like proper nutrition, staying hydrated and getting enough sleep really can help them do better on the field.