This league has 36 total Majors players (ages 11-12).
They set up three teams of 12 for their regular season.
They play each other through the season.
When it comes time to field an All Star team one ENTIRE team just so happens to be the official All Star team.
It seems to me they simply
(1) stacked the team at the beginning of the season,
(2) allowed them to practice together, and
(3) allowed them to play games together, to get ready for All Stars.