Some of the key things for young players to watch is the triangle/diamond set up for passing and the one-two's along with the fast transitions the teams are doing in the group stage. On free kicks and cornerkicks you can see the set pieces in motion. You may also see where most of the game is played (sometimes it is different for each half). Know which team had longer possession of the ball, what GKs do to keep the ball out of the goal, and watch how the players typically shoot to the opposite corner of the goal from where they are taking the shot.
It is especially good to watch the teams that are almost always in motion. Follow the players that do not have the ball, watch how the midfielders and forwards work in tandem on attacking, and how the outside defenders (cornerbacks) run down on offense then quickly get back on defense to help out the inside defenders (fullbacks) and the defensive midfielder (stopper) when the change of possession happens. All of this is a learning experience for youth players.
Lastly, FS1 also has a show they are running called "Phenoms" that I would highly recommend showing your player(s).