At 12U, the focus should still be on long-term player development and building confidence. Shortening the bench does not accomplish either goal, so providing equal playing time should be paramount.
Players at this age aren’t pro athletes. These are kids (families) who are paying to play. How do kids improve if they are not playing? Teams are generally only as strong as their weakest players, so increasing every player's ability will improve long-term player development and help improve the team's overall performance during the season.
It's imperative that youth coaches communicate their (equal) playing-time philosophy with parents and players at the beginning of the season, and reinforce it throughout. Coaches and parents should put development ahead of short-term winning in youth hockey games. Remember, these games are for the player’s development, not for the adult’s satisfaction.
In the end, youth sports are about having fun, teaching healthy life skills, instilling a strong work ethic and helping players reach their full genetic athletic potential. This requires opportunities to practice and compete in a healthy and supportive environment. It's the job of the coach to prepare every player for game-time scenarios. Players do not rise to the occasion; they fall to the level of their training. So make sure every player gets a high level of training and equal playing time.