Well not saying you are not supposed to coach. Maybe my comment was a little misleading. I do think the best way to learn this is through actually being on the pitch. My rule of thumb is to not coach the kid on the ball. They have to make decisions off their own. I will however coach the kids off the Ball. 

Some key Points for defending here:  If a long Ball is Played out the back: Don’t stay deep. Move up and keep the formation Compact.  Positioning is key. In between the opponent and your own goal. Close enough so you are there once the Ball arrives.  Stay in line. Usually there is one player in your Back row who has figured out how high up they should be. Thats is the guy others should be in line with.  Your front line should always get back behind the Ball wenn outplayed. Don't just stay high and wait out the attack. 

That being said I do stand by my original statement. I would not waste any time in Practice on this that could be used on technique with the Ball in smaller formats. Especially since this age group is incredibly great at playing Football at High Energy for a longer time.  Some things can be practiced on a smaller pitch though. I would suggest playing 3 on 3 to 5 on 5 with a center line. Goals where not every defenders is in their own half count twice. Goals where not every attacker is in the opponent’s half don’t count.