Andy, I think you can get the most help from the coaches of the team you are considering as they work with new shooters every day.
My thoughts are:
In clays shooting gun fit is king. A fancy trap gun works no better than an 870 pump unless it fits the shooters body correctly.
Length of the stock (length of pull) distance from the thumb of the grip hand to the nose is the best indicator. Slightly long is better than too short.
Position of the dominant eye. It needs to be determined which eye is dominant if the desired two eye open technique will be effective.
The dominant eye is the rear sight of a shotgun. It needs to be centered on the rib and the head should not be canted or tilted to get there. This establishes horizontal point of impact (left and right)
Comb height establishes the height of the eye and determines vertical point of impact ( high or low)
All trap targets rise unless they are fired on late in the flight path, so trap guns are set up to shoot high. This allows for less visual lead to be applied and allow for full view of the targets when firing.
If you can start out with a well fitting shotgun, most kids are naturals at just shooting where they look and successfully breaking targets because their hand/eye coordination is so good.
I think it is critical that successful target breaking early in the process be achieved or the kids lose confidence and desire, so that is why I think focus on gun fit is really the easiest point to start at.
More advanced techniques include stance, foot position and how it changes from post to post, equipment choices like vests, shooting glasses that have colors that enhance targets visibility , a padded rest to place the muzzle on between shots ( decreases fatigue) Increased shotgun weight can reduce recoil effects, but they get heavy during an event.
Light 1 oz target loads are good for beginners ( 8 shot is fine) but don't skimp too much on ammo.
Promo 7/8th oz dove load generally don't break clays very consistently and have sub par shot in them
