Having a couple of set plays is always good, but never make them specific to a player.
In juniors, as with seniors, in general, both teams get the same opportunity to make a shot on goal. Each possession is either going to end in a successful shot on goal, un-successful shot on goal, or a turnover. A foul is just a "pause" in that possession. An offensive rebound can be seen as a new possession, or just a "pause". Turnovers can be due to skill error (bad passing, double dribble, travel, out of bounds etc), or a forced error due to good defence.
Obviously a turnover is a failure, so get those under control first.
Shooting % and good shot selection is the key to winning games. If your team is putting up plenty of shots, and not scoring, then they need to work on that between trainings.
Motion offence is good to run and gives every one a chance, but the players need to know the basic rules around it, including spacing, screens, cutting etc. Defensively, players need to know the basic rules as well. Obviously when in defence you want to force a bad shot, or a turnover from the other team.
In U12/14 the key really is skill development, so obviously dribbling (with both hands), passing and shooting is essential. Rebounding (and boxing out) is critical for forwards as rebounding means you end up with even possessions, and defensive pressure is critical for guards as they force turnovers.
As far as set plays are concerned, I think it's good to get a couple of "standard ones". A good end baseline inbounds play is always important as you could be down by 4 and need a couple of quick goals. Same too with a sidelines inbound, sometimes you need a quick goal.
Bottom line, have a few plays up your sleeve for special times, but make sure your team can nail free throws, dribble with both hands, pass accurately, and rebound.