The "right" mouthpiece is the one that works best for THAT kid on THAT horn.
I start kids on 5C. If they have particularly full lips, they may swap for a 5B. Once they have a developed embouchure, they may move to a 3C at some point. My top level HS players (Region level players) may switch to a 1.5C at some point.
I've also had good luck with Denis Wick, Schilke, and other mouthpiece brands. It depends ENTIRELY upon the player.