Darren at under 13s simple is best try this if you have a big forward(or the bigest player on your team)have the scrumhalve with the ball on the mark facing your pack standing about ten yards back have the big runner(followed by the pack) run towards him from blind to open giving a huge shout as he taps the ball.This is a dummy run have the scrumhalve then pass to a quick player on a run up the blind e.g flanker or blind side winger. this has worked well for me when i was coaching 13s 14s 15s the opposition pack all move to the openside trying stop the big dummy runner best of luck mate neil