A growing number of private investment firms are pouring millions into youth sports, and critics say their capitalization on the athletic development of young people across the country is going too far. Each year, more multibillion-dollar firms are buying up the most competitive tournaments for popular sports like soccer, hockey and baseball.   In many cases, they are also erecting state-of-the-art training facilities, hiring top-level coaches and charging parents thousands in exchange for their children being able to participate. The increasingly lucrative…